Science Questions

Science Question 1

Lakin Jones — Sun, 01 Apr 2018 15:00:38 +0000

‘How does the magnetic field in the solar wind source regions connect to the photosphere and the heliosphere?’

Studies of streamers are the primary means for addressing this question, as they are relatively steady structures from which the slow solar wind is thought to emanate. In-situ observations of the magnetic fields and plasma properties of these structures along the 1D spacecraft trajectory will be combined with remote white light observations by WISPR. Previous insitu observations from outside 0.3 AU have been extrapolated back to the Sun, indicating that the slow wind may originate in these streamers and the closed magnetic fields below them (e.g. Gosling et al. 1981). This conclusion has been supported by remote sensing of the slow wind, for example by interplanetary scintillation measurements from Voyager 2 (Woo and Martin 1997) and via SOHO Ultraviolet Coronal Spectrometer (UVCS; Kohl et al. 1995) Doppler measurements combined with context images from the Large Angle and Spectrometric Coronagraph (LASCO; Brueckner 1995) coronagraphs (Habbal et al. 1997). SPP will for the first time allow us to definitively test this inference. WISPR will image streamers as SPP approaches them, giving a highly accurate measure of when and through which part (i.e., edge, center) the spacecraft flies. The in-situ observations will then measure the plasma properties and the magnetic field in and around that streamer, telling us what the solar wind characteristics are in the streamer, and how they vary across the streamer. WISPR will address fundamental questions about the structure of streamers: Are streamers the folds of a single current sheet encompassing the Sun or are there multiple current sheets which create multiple streamers? Does this structure change from solar minimum to solar maximum? What is the internal structure of streamers? High-resolution WISPR observations on par with those of the LASCO and SECCHI coronagraphs will put the in-situ measurements into context. The current coronagraphs observe streamers globally but are unable to measure their 3D structure at resolutions better than 14◦ (the rate of solar rotation from 1 AU). In contrast, the SPP orbits result in up to 10°ø faster sweeps around the Sun thus enabling streamer 3D tomographic reconstructions from the WISPR images with spatial resolutions of ∼1◦. These reconstructions will allow us to investigate the structures that comprise the heliospheric plasma sheet (HPS) and to study the relation of the HPS to the heliospheric current sheet (HCS), while the SPP in-situ magnetic field measurements will determine the presence or absence of current sheets inside streamers.

WISPR Science Requirements Traceability Matrix
Science Objective	1. Trace the flow of energy that heats and accelerates the colar corona and solar wind.			2. Determine the structure and dynamics of plasmaand magnetic fields at the sources of the solar wind.				3. Explore mechanisms that accelerate and transport energetic particles.		G1. Explore dusty plasma phenomena in the near-Sun environment and their influence on the solar wind and energetic particle formation.
Science Question#	1	2		3	4		5	6	7	8	9
Science Measurement Objective	Velocity and brightness evolution of small scale features in coronal holes and streamers	Location, morphology of the stream interfaces boundaries, and wave turbulance		Map morphology of coronal structure to SPP orbit	Velocity and brightness evolution of small scale features in coronal holes and streamers		Velocity, acceleration and mass density of evolving structures	Location, morphology and speed of shocks near the Sun	Morphology, velocity, acceleration of CMEs and shocks out to SPP Orbit	F-corona brightness, morphology and variability as a function of heliocentric distance	Location, morphology of CMEs and coronal evolution along SPP orbit
Science Measurements	H-t and mass measurements of solar wind features	H-t and mass measurements of solar wind features	Power spectra of density fluctuations at different heliocentric distances	Images of coronal and heliospheric solar wind structures in visible	H-t and mass measurements of solar wind features	Density power spectra at various heliocentric distances	H-t and mass measurements of solar wind features	High cadence height-time plots and density measurements of CME fronts	High cadence height-time plots and density measurements of CME fronts	Images of F-corona during the orbit	Images and height plots of the corona ahead of SPP passage
Observation Requirements
Scene Radial Coverage	14º-90º		14º-20º	14º-90º		14º-20º	14º-90º	14º-60º	14º-90º	14º-90º	14º-90º
Scene Transverse Coverage	25º-55º		12º	25º-55º		12º	25º-55º	25º-55º	25º-55º	25º-55º
Scene Latitude Coverage	-45 to +40º⁵		-25º to +15º	-45 to +40º⁵		-25º to +15º	-45 to +40º⁵	-45 to +40º⁵	-45 to +40º⁵	-45 to +40º⁵
Image Spatial Resolution (arcmin)	≤ 6.4^1,2,3 to ≤ 25.6^4b		≤ 6.4	≤ 6.4^1,2,3 to ≤ 25.6^4b		≤ 6.4	≤ 6.4^1,2,3 to ≤ 25.6^4b	≤ 6.4	≤ 6.4	≤ 6.4^1,2,3 to ≤ 25.6^4b
Photometric Sensitivity (Signal-to-Noise-Ratio)	≥ 20^1,2, ≥ 5^3,4		≥ 20	≥ 20^1,2, ≥ 5^3,4		≥ 20	≥ 20^1,2, ≥ 5^3,4	≥ 20^1,2, ≥ 5^3,4	≥ 20^1,2, ≥ 5^3,4	≥ 20^1,2, ≥ 5^3,4
Cadence (min)	≤ 40^1,2,3 ≤ 80⁴	≤ 2.5^1a to ≤16.5^4a ≤ 4.5^1b to ≤40^4b ≤9.0^1c to ≤80^4c	≤ 4 sec	≤ 40^1,2,3 ≤ 80⁴	≤ 2.5^1a to ≤16.5^4a ≤ 4.5^1b to ≤40^4b ≤9.0^1c to ≤80^4c	≤ 4 sec	≤ 18^1,2 ≤ 40³ ≤ 80⁴	≤ 5	≤ 5^1a,1b ≤ 10^1c	≤ 80	≤ 40^1,2,3 ≤ 80⁴
Baseline Mission Observing Period (days)	240		15	240		15	240	240	240
¹0.046-0.07 AU ^1a14º-39º for 0.046-0.07 AU ^1b39º-69º for 0.046-0.07 AU ^1c69º-90º for 0.046-0.07 AU		²0.07-0.11 AU ³0.11-0.174 AU ^3a14º-39º for 0.11-0.174 AU		⁴0.174-0.25 AU ^4a14º-39º for 0.174-0.25 AU ^4b39º-69º for 0.174-0.25 AU ^4c69º-90º for 0.174-0.25 AU		⁵at 14º ^*10 min image sequences per hour

Comparison of the SECCHI/HI observation of solar wind structures (image) to the heliospheric current sheet (red surface) predicted by an MHD model. The meridional slice is the model solar wind velocity (Vourlidas and Riley 2007)

WISPR will image the extension of streamer structures far into the heliosphere and compare their measured location and densities to in-situ measurements and coronal models. The SECCHI/HI observations have shown that this is possible. In Fig. 3, taken from Vourlidas and Riley (2007), the location of the HCS, based on an MHD simulation, is projected onto a 2-hour SECCHI/HI running difference image showing quiescent solar wind structures. The figure shows that the largest intensity, therefore density, variability corresponds to locations nearest the HCS. These measurements can identify the sources of the solar wind structures when compared with in-situ abundance measurements from SPP, Solar Orbiter, and Earth-orbiting spacecraft. WISPR will have much better sensitivity and spatial resolution than any other heliospheric imager to date (Table 1). Thus, WISPR images will trace the HPS boundaries, their evolution and their relation relative to the HCS in much greater detail than possible with STEREO. When combined with the in-situ observations from the SPP and other missions (e.g. Solar Orbiter), the WISPR observations will provide strong constraints on the origin and evolution of the solar wind plasma in the heliosphere.

Science Questions

WISPR Pub Number 2

Science Question 2

Lakin Jones — Sun, 01 Apr 2018 09:05:05 +0000

‘How do the observed structures in the corona evolve into the solar wind?’

(As discussed in Question 1) Streamers are expected to be the source of the slow solar wind. How they provide this slow wind, however, has not yet been proven, though a number of models of slow solar wind acceleration have been proposed. For example, the models and simulations presented by Einaudi et al. (1999, 2001) show that the slow solar wind can be accelerated in streamers via coupling to the fast solar wind on either side of the streamer current sheet. Tearing modes and Kelvin-Helmholtz modes in the streamer create islands, which are then accelerated by the nearby fast wind (see, e.g., Rappazzo et al. 2005). Antiochos et al. (2007), on the other hand, suggest that the slow solar wind may be accelerated by continuous small-scale reconnection events, which occur between closed and open magnetic fields at the boundaries of coronal holes. WISPR will look for signatures of these mechanisms by observing and characterizing structures, which are ejected into the solar wind from streamer current sheets.

White-light imaging with the LASCO coronagraphs has revealed a variety of such dynamical phenomena within the HPS in the outer corona, including plasma blobs that are ejected continually from the cusps of streamers (Sheeley et al. 1997; Wang et al. 1999b; Wang and Sheeley 2006), ray-like structures pervading the streamer belt (Thernisien and Howard 2006), and swarms of small-scale inflows (Wang et al. 1999a; Sheeley and Wang 2001) that occur during times of high solar activity (Fig. 4). The helmet streamers in which these structures are created comprise open field lines lying over closed magnetic loops. Reconnection between open and closed magnetic field lines (Antiochos et al. 2007; interchange reconnection: Crooker et al. 2004; Zurbuchen et al. 2002), between closed magnetic fields lines (generating helical fields) and between open field lines of opposite polarities (Einaudi et al. 1999; Wang et al. 2007; Linton et al. 2009) have all been invoked as the different mechanisms which could trigger the formation and release of such streamer blobs. In addition to serving as a potential source of the slow solar wind, these reconnection processes have a bearing on questions as diverse as the formation and evolution of the HPS/HCS, the heliospheric magnetic flux budget, the solar-cycle evolution of the coronal field, and the rigid rotation of coronal holes. To investigate these phenomena and to test slow solar wind models, we need detailed velocity profiles using high cadence WISPR measurements of streamer outflows, correlated with the in-situ measurements. WISPR observations are essential for studying reconnection in the high corona by providing the 3D location and morphology of streamer ejections and measurements of their evolution before the SPP in-situ payload intercepts them.

To study the details of these small-scale transients, we need high-resolution observations, as these transients commonly take up only a few pixels in current LASCO and SECCHI coronagraph observations (Sheeley et al. 2008; Rouillard et al. 2008, 2009). WISPR is designed with a 6.4 arcmin resolution so that it can image and trace the streamer blobs, within its FOV, to large heights and with a resolution equivalent to or better than that of the LASCO or SECCHI coronagraphs. The increased resolution and sensitivity of WISPR due to the much smaller contribution of the F-corona brightness (Sect. 1.4) will reduce the scatter in the outer velocity measurements. With the combined WISPR and in-situ measurements, we will determine how the slow solar wind densities and speeds vary across the streamer and how that depends on the current sheet structure. WISPR’s wide FOV enables the measurement of the true velocity and acceleration profiles of the transient slow solar wind flows and determine accurately the mass flux contriThe Wide-Field Imager for Solar Probe Plus (WISPR) bution of blobs and other ejections to the solar wind. This will provide, for the first time, quantitative tests of the various theoretical models, which explain the origin of the slow solar wind. We will be able to determine if the slow wind is accelerated by viscous coupling to the fast wind just outside the streamer, if it is self-accelerated by turbulence and reconnection within the streamer or if it is accelerated by reconnection in the corona at the boundary between the streamers and coronal holes. The determination of the local structure of the solar wind as it correlates with the streamer observations is only the first step in understanding the full solar wind geometry. These measurements must then be combined with high-resolution tomographic reconstructions of the transient features, which originate in streamers. This will vastly improve our ability to determine the location, size and propagation direction of these streamer transients. By following the evolution of these transients, we will be able to determine the 3D flows and mass fluxes around streamers and the degree to which these flows are non-radial below the sonic point. Combining these WISPR remote observations with the in-situ observations will give us the exciting new capability to reconstruct a significant part of the slow solar wind outflow, providing new insights into the structure of the corona and key inputs for models of coronal fields and solar wind acceleration.

In-situ observations reveal significant fine-scale structure within the fast solar wind which led Feldman et al. (1996) to surmise that these structures are remnants of reconnection events back in the solar corona (e.g., jets, spicules). However, the origin of these fast solar wind structures is unknown because line-of-sight effects and the reduced density within coronal holes hinder the imaging of the fine scale structures from 1 AU, especially for equatorial coronal holes. The proximity of the SPP orbit to the solar corona essentially removes the effects of the F-corona and reduces the number of overlapping structures along the line of sight (LOS) (Sect. 1.4). It provides a unique opportunity to detect and image the faint plasma within coronal holes. WISPR will be able to image this plasma from both equatorial and polar coronal holes up to a heliolatitude of ∼40◦ or higher depending on the solar B angle. WISPR will detect the plumes with higher contrast and spatial resolution than has ever been possible. It will measure the plume/interplume density variations and determine the presence of fine scale structure within coronal holes, thus allowing precise measurements of the contribution of plumes and interplume regions to the observed fast wind mass flux. WISPR will be able to image the fast wind for the first time and track such blobs, if they exist, within polar plumes. WISPR will provide these crucial observations over a significant part of the solar cycle. Hence, we will obtain the first detailed measurements of the fast wind acceleration profile over large areas of the corona and, with the addition of the SPP in-situ data, provide important constraints for testing theories of fast solar wind acceleration. Together with the slow solar wind observations discussed above, these studies will form comprehensive sets of observations, which will substantially improve our understanding of the sources of the slow and fast solar wind. These measurements will be invaluable as initial condition inputs to the real-time large-scale heliospheric models such as ENLIL and will lead to improved forecasting for space weather conditions at Earth and other planets. On larger scales, the solar wind flow is disrupted by CMEs. WISPR will contribute to CME studies in two ways. First, the high-resolution WISPR tomographic images will allow us to recreate the 3D structure within CMEs. Second, when combined with in-situ measurements of magnetic field and plasma properties, the WISPR observations will allow us to determine the physical state of the ejected CME plasma (thermal, magnetic and kinetic) right at the initial boundary of most CME propagation models (e.g., ENLIL) which will greatly enhance their performance and improve forecasting capabilities. In addition, most

LASCO speed measurements of streamer blobs. The WISPR fields of view for two perihelia are also shown.

Periodicities in the solar wind density derive from SECCHI/HI observations. Left: Tracing of individual density blobs within a streamer from 15 to 60 Rs. Right: The derived periodicities of 5 h.

Science Questions

Science Question 3

Lakin Jones — Sat, 10 Mar 2018 18:41:25 +0000

‘Is the source of the solar wind steady or intermittent?’

Various in-situ studies have suggested that the inner heliosphere is filled with a network of entangled magnetic flux tubes and that the flux tubes are fossil structures that originate at the solar surface (e.g., Zaqarshvili et al. 2014; Borovsky et al. 2008). The tube walls are associated with large changes in the ion entropy density and the alpha-to-proton ratio. The median size of the flux tubes at 1 AU is 4.4 °ø 105 km (Borovsky 2006; Borovsky et al. 2008). The magnetic flux in the tubes at 1 AU corresponds to the magnetic flux in field concentrations in the photospheric magnetic carpet. Using 11 years (1995–2005) of solar wind observations from the Wind spacecraft, Viall et al. (2009) showed that periodic proton density structures occurred at particular radial length scales more often than others. An analysis of the alpha to proton solar wind abundance ratio variations strongly suggests that these periodic solar wind density structures originate in the solar corona. Some recent models of abundance variations predict that they are set in the chromosphere (Laming 2009). Because the observed emission is related to the number of electrons along the LOS, intensity variations provide a direct measure of solar wind density variations, which can be compared to Earth-based interplanetary scintillation or SPP in-situ measurements. Viall et al. (2010) have identified specific periodicities by following individual blobs of <1200Mm size through the SECCHI/HI FOV (Fig. 5). The minimum size that could be measured is determined by the cadence and exposure times of the instrument (40 min and 30 min, respectively for HI-1). Our analysis of density data from the SECCHI/HI suggests that we can obtain measures of the fine-scale solar wind variability directly from the WISPR images down to length scales of ∼11 Mm at closest perihelion. This estimate is scaled from the results in Viall et al. (2010) using the expected cadence for WISPR (4 s).

Science Questions

WISPR Pub Number 1

Science Question 4

Lakin Jones — Thu, 15 Feb 2018 15:58:59 +0000

‘How is energy from the lower solar atmosphere transferred to, and dissipated in, the corona?’

While the answers to these questions require detailed in-situ observations of the plasma and magnetic field in the inner corona, the imaging observations by WISPR can provide essential information to assist the interpretation of the in-situ data. There is the possibility that small-scale reconnection heats and accelerates the solar wind. If such reconnection is an important contributor to solar wind heating, then in-situ evidence of such events, such as abrupt velocity and magnetic field changes (Gosling et al. 2007) and energetic particles should be quite common. However, tracing their origins (lower atmosphere or the outer corona) using extreme ultraviolet (EUV) or white light imagers on distant platforms (such as SDO or Solar Orbiter) will be difficult due to the small spatial scales involved. By providing high resolution and high dynamic range imaging on the ram-side, WISPR will observe the intermittent solar wind, which is intercepted later by the SPP in-situ instruments. Subsequent joint in-situ/imaging analysis on the ground will clarify which, if any, of the observed outflow structures are results of reconnection. The WISPR images can then be compared to coronagraph and EUV imaging from other spacecraft to allow tracing of such features lower in the solar atmosphere.

WISPR Pub Number 1

Science Questions

Science Question 5

Lakin Jones — Mon, 15 Jan 2018 16:05:06 +0000

‘How do the processes in the corona affect the properties of the solar wind in the heliosphere?’

While the slow wind appears to originate in streamers, the fast wind originates in the open magnetic fields of coronal holes. The Helios observations revealed that the latitudinal/ longitudinal edges of the high-speed solar wind streams from coronal holes are very sharp (Schwenn 1978), with gradients of 100 km/s/deg near 0.3 AU. The sharp edges are less apparent in the Ulysses and near-Earth data perhaps due to interplanetary dispersion on the trailing edges (the fastest plasma runs away from the slower plasma immediately behind it) and because of the change in profile on the leading edges. In contrast to Helios observations, theWang and Sheeley (1990) numerical model of the solar corona, which relates the expansion of magnetic flux tubes to the speed of the solar wind by assuming that the slow solar wind originates on the boundary of coronal holes, suggests that the latitudinal/longitudinal edges of streams near the Sun are broad regions with gradients of 20 km/s/deg.

WISPR observations will be able to clarify this debate, as it will image the change from low to high-density plasma that marks the transition from high to low speed solar wind. High-resolution white-light images by WISPR will be obtained inside 0.25 AU where, according to Parker spiral theory, the interface between fast and slow solar wind streams will be viewed edge-on. The boundary will appear as a brightness gradient, steepening slowly with increasing heliocentric distance. WISPR images will measure the thickness of the brightness gradient directly and, by tracking its co-rotation over several days, will determine its 3D topology and temporal evolution. Additionally WISPR will pass through the stream interfaces near 10 Rs and in-situ observations of the boundary thickness will be compared with white-light observations.

Estimation of the breakpoint frequency between injection and inertial scales as a function of heliocentric distance based on Helios observations. The simple fit to the three points shows a breakpoint frequency at ∼0.2 Hz at 9.5 Rs, easily accessible by WISPR. Inset: The magnetic field spectra used for the breakpoints (Bruno and Carbone 2005)

Turbulence is another way the corona affects the solar wind properties. Turbulent cascade, widely accepted as a mechanism for the generation of ion-cyclotron waves, has good theoretical and observational support (Hollweg 2008). However, the solar wind, and consequently its turbulence levels, evolves as the wind propagates away from the Sun, thus confusing or diluting signatures of the low corona acceleration processes and of the original wave spectrum. Energy is injected at low frequencies varying from days to months and cascades with a Kolmogorov power spectrum of f −5/3. Helios observations have shown that the breakpoint between the inertial and injection scales moves to higher frequencies closer to the Sun but the injection power spectrum maintains the f −1 spectrum (Fig. 6). The source of the f −1 spectrum is still under debate. Matthaeus and Goldstein (1986) have suggested that it originates from reconnection events in the corona and hence indicates the influence of reconnection in coronal heating. These results are based on solar wind velocity and magnetic field fluctuations. The density fluctuations are harder to interpret. At 1 AU, there is evidence of both turbulence and coherent structures contributing to the observed fluctuations (Viall et al. 2009). To separate them and trace their origins, two-dimensional imaging observations are required. In-situ density spectra exhibit f −1 and f −5/3 spectra (Marsch and Tu 1990) in close correspondence to magnetic field spectra, but they also exhibit 1/f 2 spectra.

We have only a basic idea of whether this behavior persists closer to the Sun. The main information is provided by density power spectra using interstellar scintillation (e.g., Coles and Harmon 1989) but the relation of the density fluctuations to ion-cyclotron waves is unclear and radio observations near the Sun are rare due to the lack of suitable radio sources and dedicated solar radio instruments. Recently, Bemborad et al. (2008) obtained remote imaging spectra with 1/f and 1/f 2 behavior in the Lyα line using SOHO/UVCS observations. However, the long integration times of 300 s, required to obtain the necessary sensitivity, restricted their study to low frequencies away from the spectral breakpoint. Such studies are further restricted by line-of-sight effects and uncertainties in the origin of the Lyα emission. However, they demonstrated the power of remote imaging by simultaneously obtaining spectra over a variety of longitudes, latitudes and heliocentric distances.

The SPP orbit offers many advantages for the pursuit of such measurements with WISPR based on our experience with the SECCHI/HI performance on solar wind structures. First, the proximity of SPP to the coronal structures allows much higher contrast observations with higher cadence than is possible from 1 AU. Second, the spectral breakpoint between injection and inertial scales is expected to drift from 100 s at 40 Rs down to 5 s at 9.5 Rs based on a simple extrapolation of the Helios measurements (Fig. 6). Both of these time-scales are easily within the WISPR capabilities. We have designed a specific WISPR observing program for this case. For example, prior to each solar encounter, we will use synoptic images from WISPR or other coronagraphs to predict when SPP will cross a solar wind structure of interest (e.g., an HPS boundary or a fast stream interface). For a specified time interval during the SPP perihelion (currently 10 min every hour), WISPR will obtain images over a restricted FOV around the region of interest with extremely high cadence (up to 1 s). A power spectrum of the density fluctuations can then be constructed with variable cadences for direct comparison to similar spectra obtained by the FIELDS instruments on SPP. WISPR will provide density power spectra at or below the spectral break between inertial and injection scales, even at the nearest perihelion approach. WISPR will provide many simultaneous spectra for different coronal structures and will monitor their evolution. When combined with the tomographic information from the synoptic images, the WISPR turbulence program will be a major enhancement to the turbulence measurements from the SPP in-situ instruments resulting in a much more robust understanding of the near-Sun turbulence.

WISPR Pub Number 1

Science Questions

Science Question 6

Lakin Jones — Fri, 12 Jan 2018 16:29:54 +0000

‘What are the roles of shocks, reconnections, waves, and turbulence in
the acceleration of energetic particles?’

CME-driven shocks play a central role in determining the energetic particle populations in the heliosphere and in driving geospace storms. They are known to accelerate solar energetic particles (SEPs) to high energies (e.g., Reames 1999; Kahler 2001), even GeV energies (Bieber et al. 2004) during the so-called gradual SEP events. Fermi acceleration is the likely acceleration mechanism for quasi-parallel shocks while gradient-drift acceleration operates at quasi-perpendicular shocks (e.g., Lee 2000). The geometry of the shock seems to play a further role in the observed variability of the spectral characteristics and composition of SEPs (Tylka 2005). The shock compression ratio determines the power law index of the SEP spectrum under some simplifying assumptions such as equilibrium conditions. It appears that the particle kinetic energy might be a fairly significant percentage of the CME kinetic energy (Mewaldt et al. 2005). Many of these shock-related parameters (geometry, compression ratio, speed) are available or can be deduced from in-situ measurements at 1 AU. None, however, is actually measured in the low corona where the highest energy particles originate (≤10 Rs, Tylka 2005). Moreover, the large scatter in the correlation between CME speeds and SEP peak intensities suggests a complex interplay among the CME speed, the acceleration mechanism(s) and the ambient environment.

Some works have focused on the role of the variations of the environment through which the CME shocks and particles propagate (Gopalswamy et al. 2004; Kahler and Vourlidas 2005, 2013). The results indicate that SEP-rich CMEs tend to occur during periods of enhanced activity signifying the presence of elevated levels of seed particles. But the coronagraphic observations also show that SEP-rich CMEs tend to have much brighter fronts than A. Vourlidas et al. SEP-poor events. Since bright emission in a coronagraph image may imply a large extent along the LOS, the latter finding suggests that SEP-rich CMEs either attain larger longitudinal and latitudinal extents than SEP-poor CMEs or achieve higher compression ratios. Therefore, the height of formation of the shock, the 3D extent of the CME, and the monitoring of the activity levels (via CMEs, and jets) are necessary observations for a better understanding of the generation and propagation of SEPs.

WISPR will provide these crucial observations for SPP. The telescope will image CMEs and their associated shocks at the coronal heights where the particles originate (≤10 Rs) with high spatial and temporal resolution to resolve the locations of the CME-driven shocks, for all SPP perihelion distances (Table 1). Previous work has shown that CME-driven shocks can be easily detected in coronagraphs (Vourlidas and Ontiveros 2009) and that several physical parameters, such as density compression ratio, speed, and even upstream magnetic field, can be derived.With its higher spatial and sensitivity performance, WISPR will readily observe and characterize the evolution of even the fastest shocks. For example, the synoptic cadence of 5–10 min within 15 Rs (Table 2) will allow 13–26 observations of a 2000 km/s CME in the WISPR FOV providing detailed information on the evolution of the associated shock.

WISPR Pub Number 1

Science Questions

Science Question 7

Lakin Jones — Thu, 11 Jan 2018 16:33:45 +0000

‘How are the energetic particles transported radially across magneticfield lines from the corona to the heliosphere?’

To address this question it is important to characterize accurately the spatial extent of shocks. WISPR will be able to observe the shocks as they expand towards SPP. These observations will monitor the kinematic evolution and the interactions of the shock with the ambient environment providing crucial information for interpreting the in-situ observations of the same shock. The WISPR inner FOV extends below 10 Rs for all heliocentric distances during the science-observing window, and therefore will be able to contribute to the SEP analysis for the entirety of the SPP science operations. WISPR will be able to observe shocks and CMEs as they go over the Solar Orbiter and other inner heliospheric probes that may be operating at the time. The multipoint observations will be used to reconstruct the 3-D structure of CMEs and their associated shocks. Alternatively, the shocks can be localized with the help of type-II radio observations from FIELDS, and the corresponding instruments on Solar Orbiter and STEREO. The rapid image cadence of WISPR ensures that we will record several images of the shock and associated driver before the increased cosmic ray flux due to the accompanying SEPs raises the background noise levels too high for reliable imaging.

WISPR Pub Number 1

Science Questions

Science Question 8

Lakin Jones — Wed, 10 Jan 2018 16:35:37 +0000

‘What is the dust environment in the inner heliosphere?’

The visible emission at 1 AU, from heights above 4 Rs, is dominated by scattering from interplanetary dust, the F-corona. It is a nuisance for coronal studies in the visible as it obscures the signal from CMEs and coronal streamers. Accurate removal of the F-corona is The Wide-Field Imager for Solar Probe Plus (WISPR) essential for the derivation of coronal density structure (e.g., Hayes et al. 2001) but the current F-coronal models are unreliable, as LASCO/C3 observations have shown. The failure of the models stems from our incomplete understanding of the physical properties and distribution of the dust in the inner heliosphere. Most of what we know comes from coronagraph and eclipse observations from Earth and the in-situ and photometric observations from the Helios mission in the 1970’s (Leinert et al. 1998).

The F-corona brightness results from the line-of-sight integral of the scattering from 1–100 μm dust particles. These particles undergo efficient forward scattering at small angles. Hence dust located in the region about halfway between the Sun and the observer generates most of the F-corona brightness at small elongations (Mann et al. 2004) resulting in the very stable F-corona emission observed by LASCO. This complicates the inversion of the brightness observations and leads to unreliable determinations of the structure and density distribution of the near-Sun dust and its interplay with planets. For example, the existence of a dust-free zone in the inner corona (<4 Rs) due to sublimation, predicted by Russell (1929), has never been proven experimentally and there is only a marginal detection of a planetary dust ring from Helios observations in the Venus orbit, similar to that seen at Earth’s orbit (Leinert and Moster 2007; Jones et al. 2013). Such shortcomings have significant impact on our understanding of dust-plasma interactions and the interpretation of the evolution of circumstellar dust rings and planet formation.

WISPR will revolutionize the remote sensing study of the F-corona by going much closer to the Sun and with much higher sensitivity, spatial resolution and spatial coverage compared to the Helios photometers. Thanks to 18 years of LASCO/C3 observations, we have developed robust data analysis techniques to achieve F-corona model subtractions with accurate photometry. The same techniques are used for the removal of the F-corona from the SECCHI/HI images and the upcoming SoloHI instrument on the Solar Orbiter mission.

With WISPR we will extract quantitative measurements and record the first F-corona images from locations within 0.3 AU. During the perihelion pass, the region of dust contributing to the scattering will move closer to the Sun contributing to an increase in the brightness (due to the increased density of dust) until eventually it must start to roll over close to the Sun and finally disappear at the dust-free zone (Fig. 7). The high orbital velocities during the perihelion passages will result in brightness measurements of the F-corona from a multitude of vantage points relative to the dust cloud thus allowing us to derive much more accurate measurements of the dust density distribution within 0.3 AU. Thanks to the reduced line-of-sight effect, WISPR will be able to detect and measure the boundaries of the dust-free region and possibly verify the existence of dust enhancements in the orbits of Venus and Mercury.

Another unique science opportunity is the search for planetoids within theMercury orbit. A dynamically stable region interior to Mercury’s orbit is predicted to contain a population of small, asteroid like bodies called Vulcanoids from the early solar system and may be the source of impacts onto Mercury. Searches for the existence of Vulcanoids have not been successful. Durda et al. (2000), Merline (2008), and Steffl et al. (2013) have used LASCO, Messenger and SECCHI observations to search for Vulcanoid objects and have put upper limits on the number of objects above certain sizes. While asteroids have been detected within the Vulcanoid region (0.08–0.2 AU), none were Vulcanoids. With WISPR, we will be able to extend these searches to fainter objects and place new constraints on the formation and evolution of objects in this region.

WISPR Pub Number 1

Science Questions

Science Question 9

Lakin Jones — Tue, 09 Jan 2018 16:37:56 +0000

The predicted coronal brightness from WISPR at altitudes of 9.5 and 54.7 Rs for the equatorial F and K coronae. The photon noise was calculated assuming an exposure time of 1 s for the 9.5 Rs case and 30 min for the 54.7 Rs case. The plots show that WISPR will produce very high SNR images of the solar corona over the instrument FOV

‘What is the nature of dust–plasma interactions and how does dust modify the spacecraft environment close to the Sun?’

As discussed by Mann et al. (2004), forward scattering washes out the small-scale structure of the corona as well as any information on short-term variability within 0.3 AU from the Sun. Thus, we have no knowledge of the effects of CMEs or sungrazer comets on the dust dynamics near the Sun. WISPR will obtain the first reliable measurements of the F-corona brightness gradient within the first few degrees from the Sun and will observe the evolution of sungrazer (and other comet types) tails within its large FOV.

LASCO observations show that sun-grazing comets occur on average every 2–3 days and their brightness peaks at 10–14 Rs (Knight et al. 2010), right in the middle of the WISPR FOV during close perihelia. Although it is clear they do not survive their perihelion, the actual distance at and process through which their nucleus is disrupted remain unresolved. Most of the sungrazers dim below detection at around 7 Rs and may be completely destroyed by 3 Rs, as a handful of UVCS observations suggest (e.g., Bemborad et al. 2005). Furthermore, Kimura et al. (2002) have suggested that sungrazers should exhibit a second brightness peak at 4–6 Rs due to the sublimation of crystalline and amorphous pyroxenes. WISPR will have the sensitivity, spatial coverage, and cadence to resolve these issues albeit based on a smaller sample of comets than LASCO or SECCHI due to the SPP orbit and operational restrictions.

These comets deposit dust into the near-Sun environment but because of their highly inclined orbits, the dust from their tails must leave the ecliptic quickly. Mann et al. (2004) reached the conclusion that the sun-grazer contribution to the near-Sun dust is negligible but their estimates were based on mass and size distributions derived from SOHO measurements at 1 AU (Sekanina 2001). The actual dust flux and size distribution are unknown and analysis of theWISPR observations is required to determine accurately the contribution of sun-grazer comets to the dust environment.

As discussed above, current F-coronal models are unreliable close to the Sun, but the F-corona brightness must start to roll over, perhaps inside 0.1 AU, due to the increased radiation pressure, evaporation, and Lorentz forces acting on the particles. This effect will be readily detectable by WISPR and will further enhance the quality of the coronal imaging (Fig. 7). Additionally, the radial distances where these processes act on is a function of the particular chemical composition of the species (Mann et al. 2004). So the combination of theWISPR observations with modeling of the dust composition should allow the estimation of the size distribution of the dust in the inner heliosphere. The improvement in the clarity, sensitivity and spatial resolution of the F-corona images combined with the repeated passages over a large part of the cycle will provide the first opportunity to study the short-term (days to years) evolution of the dust and investigate whether CMEs interact in any significant way with the interplanetary dust and whether we can use this interaction to probe the CME magnetic fields, as suggested by Ragot and Kahler (2003).

WISPR Pub Number 1

Science Questions

Science Questions

Science Question 1

‘How does the magnetic field in the solar wind source regions connect to the photosphere and the heliosphere?’

Observation Requirements

Science Question 2

‘How do the observed structures in the corona evolve into the solar wind?’

Science Question 3

‘Is the source of the solar wind steady or intermittent?’

Science Question 4

‘How is energy from the lower solar atmosphere transferred to, and dissipated in, the corona?’

Science Question 5

‘How do the processes in the corona affect the properties of the solar wind in the heliosphere?’

Science Question 6

‘What are the roles of shocks, reconnections, waves, and turbulence in the acceleration of energetic particles?’

Science Question 7

‘How are the energetic particles transported radially across magneticfield lines from the corona to the heliosphere?’

Science Question 8

‘What is the dust environment in the inner heliosphere?’

Science Question 9

‘What is the nature of dust–plasma interactions and how does dust modify the spacecraft environment close to the Sun?’

‘What are the roles of shocks, reconnections, waves, and turbulence in
the acceleration of energetic particles?’