Sun-Earth Line Research Articles

Stereoscopic measurements of coronal Doppler velocities

Context. The Solar Orbiter mission, with an orbit outside the Sun-Earth line and leaving the ecliptic plane, opens up opportunities for the combined analysis of measurements obtained by solar imagers and spectrometers. For the first time different space spectrometers will be located at wide angles to each other, allowing three-dimensional (3D) spectroscopy of the solar atmosphere. Aims. The aim of this work is to prepare a methodology to facilitate the reconstruction of 3D vector velocities from two stereoscopic line of sight (LOS) Doppler velocity measurements using the Spectral Imaging of the Coronal Environment (SPICE) on board the Solar Orbiter and the near-Earth spectrometers, while widely separated in space. Methods. We developed the methodology using the libraries designed earlier for the STEREO mission, but applied to spectroscopic data from the Hinode mission and the Solar Dynamics Observatory. We used well-known methods of static and dynamic solar rotation stereoscopy and the methods of extreme ultraviolet (EUV) stereoscopic triangulation for optically thin coronal EUV plasma emissions. We developed new algorithms using analytical geometry in space to determine the 3D velocity in coronal loops. Results. We demonstrate our approach with the reconstruction of 3D velocity vectors in plasma flows along ‘open’ and ‘closed’ magnetic loops. This technique will be applied to an actual situation of two spacecraft at different separations with spectrometers on board during the Solar Orbiter nominal phase: SPICE versus the Interface Region Imaging Spectrograph (IRIS) and Hinode imaging spectrometer. We summarise how these observations can be coordinated.

Configuration of a Magnetic Cloud From Solar Orbiter and Wind Spacecraft In-situ Measurements

Coronal mass ejections (CMEs) represent one type of the major eruption from the Sun. Their interplanetary counterparts, the interplanetary CMEs (ICMEs), are the direct manifestations of these structures when they propagate into the heliosphere and encounter one or more observing spacecraft. The ICMEs generally exhibit a set of distinctive signatures from thein-situspacecraft measurements. A particular subset of ICMEs, the so-called Magnetic Clouds (MCs), is more uniquely defined and has been studied for decades, based onin-situmagnetic field and plasma measurements. By utilizing the latest multiple spacecraft measurements and analysis tools, we report a detailed study of the internal magnetic field configuration of an MC event observed by both the Solar Orbiter (SO) and Wind spacecraft in the solar wind near the Sun-Earth line. Both two-dimensional (2D) and three-dimensional (3D) models are applied to reveal the flux rope configurations of the MC. Various geometrical as well as physical parameters are derived and found to be similar within error estimates for the two methods. These results quantitatively characterize the coherent MC flux rope structure crossed by the two spacecraft along different paths. The implication for the radial evolution of this MC event is also discussed.

Coronal mass ejections observed by heliospheric imagers at 0.2 and 1 au

Context. We study two coronal mass ejections (CMEs) observed between April 1 to 2, 2019 by both the inner Wide-Field Imager for Parker Solar Probe (WISPR-I) onboard the Parker Solar Probe (PSP) spacecraft (located between about 46 and 38 solar radii during this period) and the inner heliospheric imager (HI-1) onboard the Solar Terrestrial Relations Observatory Ahead (STEREO-A) spacecraft, orbiting the Sun at about 0.96 au. This is the first study of CME observations from two viewpoints in similar directions but at considerably different solar distances. Aims. Our objective is to derive CME kinematics from WISPR-I observations and to compare them with results from HI-1. This allows us to understand how the PSP observations affect the CME kinematics, especially due to its proximity to the Sun. Methods. We estimated the CME positions, speeds, accelerations, propagation directions, and longitudinal deflections using imaging observations from two spacecrafts and a set of analytical expressions that consider the CME as a point structure and take the rapid change in spacecraft position into account. We derived the kinematics using each viewpoint independently and both viewpoints as a constraint. Results. We found that both CMEs are slow (<400 km s−1), propagating eastward of the Sun-Earth line (westward of PSP and STEREO-A). The second CME seems to accelerate between ~0.1 and ~0.2 au and deflect westward with an angular speed consistent with the solar rotation speed. We found some discrepancies in the CME solar distance (up to 0.05 au, particularly for CME #1), latitude (up to ~10°), and longitude (up to 24°) when comparing results from different fit cases (different observations or set of free parameters). Conclusions. Discrepancies in longitude are likely due to the feature that is tracked visually, rather than instrumental biases or fit assumptions. For similar reasons, the CME #1 solar distance, as derived from WISPR-I observations, is larger than the HI-1 result, regardless of the fit parameters considered. Error estimates for CME kinematics do not show any clear trend associated with the observing instrument. The source region location and the lack of any clear in situ counterparts (both at near-Earth and at PSP) support our estimate of the propagation direction for both events.

Exploring the radial evolution of interplanetary coronal mass ejections using EUHFORIA

Context. Coronal mass ejections (CMEs) are large-scale eruptions coming from the Sun and transiting into interplanetary space. While it is widely known that they are major drivers of space weather, further knowledge of CME properties in the inner heliosphere is limited by the scarcity of observations at heliocentric distances other than 1 au. In addition, most CMEs are observed in situ by a single spacecraft and in-depth studies require numerical models to complement the few available observations. Aims. We aim to assess the ability of the linear force-free spheromak CME model of the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) to describe the radial evolution of interplanetary CMEs in order to yield new contexts for observational studies. Methods. We modelled one well-studied CME with EUHFORIA, investigating its radial evolution by placing virtual spacecraft along the Sun–Earth line in the simulation domain. To directly compare observational and modelling results, we characterised the interplanetary CME signatures between 0.2 and 1.9 au from modelled time series, exploiting techniques that are traditionally employed to analyse real in situ data. Results. Our results show that the modelled radial evolution of the mean solar wind and CME values is consistent with the observational and theoretical expectations. The CME expands as a consequence of the decaying pressure in the surrounding solar wind: the expansion is rapid within 0.4 au and moderate at larger distances. The early rapid expansion was not sufficient to explain the overestimated CME radial size in our simulation, suggesting this is an intrinsic limitation of the spheromak geometry applied in this case. The magnetic field profile indicates a relaxation on the part of the CME structure during propagation, while CME ageing is most probably not a substantial source of magnetic asymmetry beyond 0.4 au. Finally, we report a CME wake that is significantly shorter than what has been suggested by observations. Conclusions. Overall, EUHFORIA provides a consistent description of the radial evolution of solar wind and CMEs, at least close to their centres. Nevertheless, improvements are required to better reproduce the CME radial extension.

On-disk Solar Coronal Condensations Facilitated by Magnetic Reconnection between Open and Closed Magnetic Structures

Abstract Coronal condensation and rain are a crucial part of the mass cycle between the corona and chromosphere. In some cases, condensation and subsequent rain originate in the magnetic dips formed during magnetic reconnection. This provides a new and alternative formation mechanism for coronal rain. Until now, only off-limb, rather than on-disk, condensation events during reconnection have been reported. In this paper, employing extreme-ultraviolet (EUV) images of the Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO), we investigate the condensations facilitated by reconnection from 2011 July 14–15, when STEREO was in quadrature with respect to the Sun–Earth line. Above the limb, in STEREO/EUV Imager (EUVI) 171 Å images, higher-lying open structures move downward, reconnect with the lower-lying closed loops, and form dips. Two sets of newly reconnected structures then form. In the dips, bright condensations occur in the EUVI 304 Å images repeatedly, which then flow downward to the surface. In the on-disk observations by SDO/Atmospheric Imaging Assembly (AIA) in the 171 Å channel, these magnetic structures are difficult to identify. Dark condensations appear in the AIA 304 Å images, and then move to the surface as on-disk coronal rain. The cooling and condensation of coronal plasma is revealed by the EUV light curves. If only the on-disk observations were be available, the relation between the condensations and reconnection, shown clearly by the off-limb observations, could not be identified. Thus, we suggest that some on-disk condensation events seen in transition region and chromospheric lines may be facilitated by reconnection.

Automated Detection of Accelerating Solar Eruptions Using Parabolic Hough Transform

Solar eruptions such as Coronal Mass Ejections (CMEs) observed in the inner solar corona (up to 4 R$_{\odot}$) show acceleration profiles which appear as parabolic ridges in height-time plots. Inspired by the white-light automated detection algorithms, Computer Aided CME Tracking System (CACTus) and Solar Eruptive Events Detection System (SEEDS), we employ the parabolic Hough Transform for the first time to automatically detect off-disk solar eruptions {from height-time plots. Due to the limited availability of white-light observations in the inner corona, we use Extreme UltraViolet (EUV) images of the Sun. In this paper we present a new algorithm, CME Identification in Inner Solar Corona (CIISCO), which is based on Fourier motion filtering and the parabolic Hough transform, and demonstrate its implementation using EUV observations taken from {\it Atmospheric Imaging Assembly} (AIA) on-board the Solar Dynamics Observatory (SDO), {\it Extreme Ultra Violet Imager} (EUVI) on-board the STEREO-A and B satellites, and {\it Sun Watcher using Active Pixel System detector and Image Processing} (SWAP) Imager on-board PROBA2. We show that CIISCO is able to identify any {off-disk} outward moving feature in EUV images. The use of automated detection algorithms, like CIISCO, can potentially be used to provide early warnings of CMEs if an EUV telescope is located at $\pm$ 90$^{\circ}$ from the Sun-Earth line, providing CME characteristics and kinematics close to the Sun. This paper also presents the limitations of this algorithm and the prospects for future improvement.

GOES-class Estimation for Behind-the-limb Solar Flares Using MESSENGER SAX

Abstract Mercury mission MESSENGER's Solar Assembly for X-rays (SAX) observed almost 700 solar flares between 2007 May 28 and 2013 August 19, as cataloged by Dennis Brian et al. The SAX instrument, part of the X-ray Spectrometer, operated at 1–10 keV, partially overlapping the energy range of the GOES X-ray spectrometers. SAX provides viewing angles different from the Earth–Sun line and can therefore be used as a GOES proxy for partially or fully occulted flares as seen from Earth. For flares with GOES classes above C2 seen on-disk for both instruments, we found an empirical relationship between the soft X-ray (SXR) fluxes measured by both SAX and GOES. Due to the different energy response of the two SXR instruments, individual events can deviate on average by about a factor of 2 from the empirical relationship, implying that predictions of the GOES class of occulted flares from SAX data are therefore accurate to within the same factor. The distinctive GOES energy response in combination with the multithermal nature of flares makes it difficult for any instrument, even other soft X-ray spectrometers, to provide a GOES proxy more accurate than a factor of 2.

Asymmetries in the Earth's dayside magnetosheath: results from global hybrid-Vlasov simulations

Abstract. Bounded by the bow shock and the magnetopause, the magnetosheath forms the interface between solar wind and magnetospheric plasmas and regulates solar wind–magnetosphere coupling. Previous works have revealed pronounced dawn–dusk asymmetries in the magnetosheath properties. The dependence of these asymmetries on the upstream parameters remains however largely unknown. One of the main sources of these asymmetries is the bow shock configuration, which is typically quasi-parallel on the dawn side and quasi-perpendicular on the dusk side of the terrestrial magnetosheath because of the Parker spiral orientation of the interplanetary magnetic field (IMF) at Earth. Most of these previous studies rely on collections of spacecraft measurements associated with a wide range of upstream conditions which are processed in order to obtain average values of the magnetosheath parameters. In this work, we use a different approach and quantify the magnetosheath asymmetries in global hybrid-Vlasov simulations performed with the Vlasiator model. We concentrate on three parameters: the magnetic field strength, the plasma density, and the flow velocity. We find that the Vlasiator model reproduces the polarity of the asymmetries accurately but that their level tends to be higher than in spacecraft measurements, probably because the magnetosheath parameters are obtained from a single set of upstream conditions in the simulation, making the asymmetries more prominent. A set of three runs with different upstream conditions allows us to investigate for the first time how the asymmetries change when the angle between the IMF and the Sun–Earth line is reduced and when the Alfvén Mach number decreases. We find that a more radial IMF results in a stronger magnetic field asymmetry and a larger variability of the magnetosheath density. In contrast, a lower Alfvén Mach number leads to a reduced magnetic field asymmetry and a decrease in the variability of the magnetosheath density, the latter likely due to weaker foreshock processes. Our results highlight the strong impact of the quasi-parallel shock and its associated foreshock on global magnetosheath properties, in particular on the magnetosheath density, which is extremely sensitive to transient quasi-parallel shock processes, even with the perfectly steady upstream conditions in our simulations. This could explain the large variability of the density asymmetry levels obtained from spacecraft measurements in previous studies.

The Polarimetric and Helioseismic Imager on Solar Orbiter

Aims.This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.Methods.SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.Results.SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission’s highly elliptical orbit.

Innovative Solar Sail Earth-Trailing Trajectories Enabling Sustainable Heliophysics Missions

The presented trajectory design study demonstrates that near-future solar sails can reach and maintain innovative science orbits about non-traditional Earth-trailing equilibrium points. The proposed configurations can enable long duration stereoscopic solar imaging while potentially alleviating telemetry requirements constraining current solar sail studies. Initial guesses for controlled periodic orbits about equilibrium points that trail the Sun-Earth line by 1.5∘-15∘ are constructed from a linearized solar sail circular restricted three body problem equations of motion. Orbits are then converged in the full nonlinear model using a high order direct collocation method. Transfers to Earth-trailing orbits are then constructed under the assumption that the solar sail is a secondary payload on a larger spacecraft en-route to the Sun-Earth L1 Lagrange point. Naturally occurring gravitational manifolds flowing towards the Lagrange point, as well as navigation data from previous Lagrange point missions, are used to generate a set of baseline trajectories for the primary spacecraft. End-to-end trajectory design and optimization is then carried out to construct time optimal solar sail transfers to each target science orbit from initial conditions constrained to lie on the carrier spacecraft’s Lagrange point trajectory. Additionally, trade studies are performed to assess how sail performance and Earth-trailing orbit selection affect transfer time of flight and instrument pointing accuracy. Results indicate that, based on current technological developments, near future solar sails are capable of station keeping about periodic Earth-trailing orbits while maintaining near-constant solar observation. Moreover, time of flights on the order of 1 year are observed for most combinations of sail performance and feasible initial conditions.

A Comparison of Flare Forecasting Methods. IV. Evaluating Consecutive-day Forecasting Patterns

Abstract A crucial challenge to successful flare prediction is forecasting periods that transition between “flare-quiet” and “flare-active.” Building on earlier studies in this series in which we describe the methodology, details, and results of flare forecasting comparison efforts, we focus here on patterns of forecast outcomes (success and failure) over multiday periods. A novel analysis is developed to evaluate forecasting success in the context of catching the first event of flare-active periods and, conversely, correctly predicting declining flare activity. We demonstrate these evaluation methods graphically and quantitatively as they provide both quick comparative evaluations and options for detailed analysis. For the testing interval 2016–2017, we determine the relative frequency distribution of two-day dichotomous forecast outcomes for three different event histories (i.e., event/event, no-event/event, and event/no-event) and use it to highlight performance differences between forecasting methods. A trend is identified across all forecasting methods that a high/low forecast probability on day 1 remains high/low on day 2, even though flaring activity is transitioning. For M-class and larger flares, we find that explicitly including persistence or prior flare history in computing forecasts helps to improve overall forecast performance. It is also found that using magnetic/modern data leads to improvement in catching the first-event/first-no-event transitions. Finally, 15% of major (i.e., M-class or above) flare days over the testing interval were effectively missed due to a lack of observations from instruments away from the Earth–Sun line.

LUCI onboard Lagrange, the next generation of EUV space weather monitoring

Lagrange eUv Coronal Imager (LUCI) is a solar imager in the Extreme UltraViolet (EUV) that is being developed as part of the Lagrange mission, a mission designed to be positioned at the L5 Lagrangian point to monitor space weather from its source on the Sun, through the heliosphere, to the Earth. LUCI will use an off-axis two mirror design equipped with an EUV enhanced active pixel sensor. This type of detector has advantages that promise to be very beneficial for monitoring the source of space weather in the EUV. LUCI will also have a novel off-axis wide field-of-view, designed to observe the solar disk, the lower corona, and the extended solar atmosphere close to the Sun–Earth line. LUCI will provide solar coronal images at a 2–3 min cadence in a pass-band centred on 19.5. Observations made through this pass-band allow for the detection and monitoring of semi-static coronal structures such as coronal holes, prominences, and active regions; as well as transient phenomena such as solar flares, limb coronal mass ejections (CMEs), EUV waves, and coronal dimmings. The LUCI data will complement EUV solar observations provided by instruments located along the Sun–Earth line such as PROBA2-SWAP, SUVI-GOES and SDO-AIA, as well as provide unique observations to improve space weather forecasts. Together with a suite of other remote-sensing and in-situ instruments onboard Lagrange, LUCI will provide science quality operational observations for space weather monitoring.

Storm-time mesoscale field-aligned currents and interplanetary parameters

Present paper studies Field aligned currents (FACs) estimated by employing Ampere's law to the magnetic field recorded by CHAMP satellite during 24 geomagnetic storms. Low-pass filtered FACs with a cutoff period of 20 s (scale size~150 km) are used to determine FAC range, which is defined as a peak-to-peak amplitude of FAC density. Thus we are considering only the strongest positive and negative FACs emerging either from Region 1, Region 2, Region 0, or substorm current wedge systems. It is known that the FACs significantly depend on the highly variable solar wind (SW) and interplanetary magnetic field (IMF) conditions and also on the processes internal to magnetospheric-ionospheric system such as substorm. The correlation analysis carried out here shows that sometimes the FAC range, correlates well with SymH, AsyH, AsyD, AL, am and Kp indices (>95% significance), but not always. The variation of the FAC range with magnetic local times shows distinctly different patterns during southward and northward IMF conditions, with peaks near dawn-dusk during southward IMF and near local noon-midnight during northward IMF. These results are in agreement with the earlier reports. However, the seasonal dependence reveals that the noon time peak is essentially associated with the summer season. We have determined a new parameter called ‘occurrence rate of FAC range ≥1 μA/m2’ and examined it under various solar wind and IMF conditions. It is found that the probability of FAC range ≥1 μA/m2 have a clear dependence on the clock angle, suggesting more frequent intensifications during southward IMF. Clear linear dependence on the cone angle demonstrates higher occurrence probability of FAC range ≥ 1 μA/m2 when the IMF is perpendicular to the Sun-Earth line (cone angle nearing 90°). All these results based on the newly defined parameters such as FAC range and probability of FAC range ≥1 μA/m2, for the storm time mesoscale FAC are consistent with the previous studies. The FAC ranges are found to have a linear dependence on the values of IMF BY, BYZ, BT and BZ, though saturation is apparent at higher values of the IMF parameters. FAC range shows distinctly different dependence for slow and fast solar wind, suggesting the importance of the composition and properties of SW in controlling the FAC strengths.

Multipoint Study of Successive Coronal Mass Ejections Driving Moderate Disturbances at 1 au

Abstract We analyze in this work the propagation and geoeffectiveness of four successive coronal mass ejections (CMEs) that erupted from the Sun during 2013 May 21–23 and were detected in interplanetary space by the Wind and/or STEREO-A spacecraft. All these CMEs featured critical aspects for understanding so-called “problem space weather storms” at Earth. In the first three events a limb CMEs resulted in moderately geoeffective in situ structures at their target location in terms of the disturbance storm time (Dst) index (either measured or estimated). The fourth CME, which also caused a moderate geomagnetic response, erupted from close to the disk center as seen from Earth, but it was not visible in coronagraph images from the spacecraft along the Sun–Earth line and appeared narrow and faint from off-angle viewpoints. Making the correct connection between CMEs at the Sun and their in situ counterparts is often difficult for problem storms. We investigate these four CMEs using multiwavelength and multipoint remote-sensing observations (extreme ultraviolet, white light, and radio), aided by 3D heliospheric modeling, in order to follow their propagation in the corona and in interplanetary space and to assess their impact at 1 au. Finally, we emphasize the difficulties in forecasting moderate space weather effects that are provoked by problematic and ambiguous events and the importance of multispacecraft data for observing and modeling problem storms.

Observation-based modelling of magnetised coronal mass ejections with EUHFORIA

Context. Coronal mass ejections (CMEs) are the primary source of strong space weather disturbances at Earth. Their geo-effectiveness is largely determined by their dynamic pressure and internal magnetic fields, for which reliable predictions at Earth are not possible with traditional cone CME models.Aims. We study two well-observed Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model, testing for the first time the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations.Methods. Using observation-based CME input parameters, we performed magnetohydrodynamic simulations of the events with EUHFORIA, using the cone and spheromak CME models.Results. Simulations show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as the result of different Lorentz forces acting within cone and spheromak CMEs, which lead to different CME expansions in the heliosphere. Such discrepancies can be mitigated by initialising spheromak CMEs with a reduced speed corresponding to the radial speed only. Results at Earth provide evidence that the spheromak model improves the predictions ofB(Bz) by up to 12–60 (22–40) percentage points compared to a cone model. Considering virtual spacecraft located within ±10° around Earth,B(Bz) predictions reach 45–70% (58–78%) of the observed peak values. The spheromak model shows inaccurate predictions of the magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line.Conclusions. The spheromak model successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events, while modelling CMEs propagating away from the Sun-Earth line requires extra care due to limitations related to the assumed spherical shape. The spatial variability of modelling results and the typical uncertainties in the reconstructed CME direction advocate the need to consider predictions at Earth and at virtual spacecraft located around it.

Mission Design Problems for the Spectrum-Roentgen-Gamma Project

The Spectrum-Roentgen-Gamma (SRG) project is intended for surveying the entire sky in the X‑ray band using two telescopes mounted onboard the spacecraft of the same name (SRG). In addition, some specially chosen areas of the sky and some radiation sources are planned to be investigated after the completion of the survey. The spacecraft was planned to be launched in 2018 into the vicinity of solar-terrestrial collinear libration point L2, using the Proton-M launch vehicle with a DM-03 block upper stage. The technology for completely surveying the celestial sphere consists in scanning by rotating the parallel axes of telescopes around the spacecraft axis, which roughly followed the direction to the Sun. The measurement data, stored onboard the spacecraft, would then be transmitted to the ground receiving stations using the mean gain antenna, the axis of which coincided with the spacecraft’s axis of rotation. This imposed some constraints on the mission design. These constraints included the permissible amplitude of the trajectory of spacecraft motion relative to the libration point in the direction orthogonal to the Sun–Earth line. In addition, the consumption of the onboard propellant was allowed only for orbit correction needed for keeping the spacecraft near the libration point. In this connection, a method was developed using the upper stage in order to decrease the mentioned amplitude down to acceptable values, in the conditions of the requirements imposed on the unit. The paper presents an assessment of its effectiveness.

Multisatellite observations of the magnetosphere response to changes in the solar wind and interplanetary magnetic field

Abstract. We employ multipoint observations of the Van Allen Probes, THEMIS, GOES and Cluster to present case and statistical studies of the electromagnetic field, plasma and particle response to interplanetary (IP) shocks observed by the Wind satellite. On 27 February 2014 the initial encounter of an IP shock with the magnetopause occurred on the postnoon magnetosphere, consistent with the observed alignment of the shock with the spiral IMF. The dayside equatorial magnetosphere exhibited a dusk–dawn oscillatory electrical field with a period of ∼330 s and peak-to-peak amplitudes of ∼15 mV m−1 for a period of 30 min. The intensity of electrons in the energy range from 31.5 to 342 KeV responded with periods corresponding to the shock-induced ULF (ultralow frequency) electric field waves. We then perform a statistical study of Ey variations of the electric field and associated plasma drift flow velocities for 60 magnetospheric events during the passage of interplanetary shocks. The Ey perturbations are negative (dusk-to-dawn) in the dayside magnetosphere (followed by positive or oscillatory perturbations) and dominantly positive (dawn-to-dusk direction) in the nightside magnetosphere, particularly near the Sun–Earth line within an L-shell range from 2.5 to 5. The typical observed amplitudes range from 0.2 to 6 mV m−1 but can reach 12 mV during strong magnetic storms. We show that electric field perturbations increase with solar wind pressure, and the changes are especially marked in the dayside magnetosphere. The direction of the Vx component of plasma flow is in agreement with the direction of the Ey component and is antisunward at all local times except the nightside magnetosphere, where it is sunward near the Sun–Earth line. The flow velocities Vx range from 0. 2 to 40 km s−1 and are a factor of 5 to 10 times stronger near noon as they correspond to greater variations of the electric field in this region. We demonstrate that the shock-induced electric field signatures can be classified into four different groups according to the initial Ey electric field response and these signatures are dependent on local time. Negative and bipolar pulses predominate on the dayside while positive pulses occur on the nightside. The ULF electric field pulsations of Pc and Pi types produced by IP shocks are observed at all local times and in the range of periods from several tens of seconds to several minutes. We believe that most electric field pulsations of the Pc5 type in the dayside magnetosphere at L<6 are produced by field line resonances. We show that the direction of the shock normal determines the direction of the propagation of the shock-induced magnetic and plasma disturbances. The observed directions of velocity Vy predominately agree with those expected for the given spiral or orthospiral shock normal orientation.

Solar Physics From Unconventional Viewpoints

We explore new opportunities for solar physics that could be realized by future missions providing sustained observations from vantage points away from the Sun-Earth line (SEL). These include observation from the far side of the Sun, at high latitudes including over the solar poles, or from near-quadrature angles relative to the Earth (e.g., the Sun-Earth L4 & L5 Lagrange points). Such observations fill known holes in our scientific understanding of the three-dimensional time-evolving Sun and heliosphere, and have the potential to open new frontiers through discoveries enabled by novel viewpoints.

High Latitude Magnetic Impulse Events (MIEs): Polarization Peculiarities and Excitation Conditions

Polarization characteristics (polarization type, ellipticity e, tilt angle τ of the polarization ellipse’s major axis) of high-latitude magnetic impulse events (MIEs) observed at the latitude of the dayside polar cusp are studied. It is established that all impulses are elliptically polarized, being right-polarized in 43% of cases (R-type) and left-polarized in 57% of cases (L-type). The right-polarized MIEs on the ground are more pronounced in the azimuthal direction, whereas the left-polarized events are more clearly marked in the meridional direction. The MIEs of both polarization types have the properties of intermittent processes. It is shown that diurnal and seasonal variations in the occurrence frequency and amplitudes of the events depend significantly on the type of their polarization. The R- and L-type impulse events are predominantly observed during the descending and ascending phase of the solar cycle, respectively. Solar wind high-speed streams (HSSs) are more favorable for exciting right-polarized impulses, whereas left-polarized impulse events are more efficiently excited by coronal mass ejection (CME). It is established that R-type impulses emerge in the conditions when the orientation of the interplanetary magnetic field vector is close to the radial direction against the development of moderate magnetospheric substorms whereas the L-type impulses appear when IMF is perpendicular to the Sun–Earth line in the absence of substorms. The behavior of the characteristics of impulse events significantly depends on the value of the IMF Bz-component and on the angle θxB = arccos(Bx/B). It is conjectured that excitation of the two groups of impulses is caused by the IMF structures in the solar wind stream with the characteristic configuration in the ecliptic plane, which determine the polarization type and properties of MIEs.

On the Spatial Coherence of Magnetic Ejecta: Measurements of Coronal Mass Ejections by Multiple Spacecraft Longitudinally Separated by 0.01 au

Abstract Measurements of coronal mass ejections (CMEs) by multiple spacecraft at small radial separations but larger longitudinal separations is one of the ways to learn about the three-dimensional structure of CMEs. Here, we take advantage of the orbit of the Wind spacecraft that ventured to distances of up to 0.012 au from the Sun–Earth line during 2000–2002. Combined with measurements from the Advanced Composition Experiment, which is in a tight halo orbit around L1, the multipoint measurements allow us to investigate how the magnetic field inside magnetic ejecta (MEs) changes on scales of 0.005–0.012 au. We identify 21 CMEs measured by these two spacecraft for longitudinal separations of 0.007 au or more. We find that the time-shifted correlation between 30 minute averages of the non-radial magnetic field components measured at the two spacecraft is systematically above 0.97 when the separation is 0.008 au or less, but is on average 0.89 for greater separations. Overall, these newly analyzed measurements, combined with 14 additional ones when the spacecraft separation is smaller, point toward a scale length of longitudinal magnetic coherence inside MEs of 0.25–0.35 au for the magnitude of the magnetic field, but 0.06–0.12 au for the magnetic field components. This finding raises questions about the very nature of MEs. It also highlights the need for additional “mesoscale” multipoint measurements of CMEs with longitudinal separations of 0.01–0.2 au.