Solar Magnetospheric Coordinates Research Articles

Investigation of the relationship between geomagnetic activity and solar wind parameters based on a novel neural network (potential learning)

Predicting geomagnetic conditions based on in-situ solar wind observations allows us to evade disasters caused by large electromagnetic disturbances originating from the Sun to save lives and protect economic activity. In this study, we aimed to examine the relationship between the Kp index, representing global magnetospheric activity level, and solar wind conditions using an interpretable neural network known as potential learning (PL). Data analyses based on neural networks are often difficult to interpret; however, PL learns by focusing on the “potentiality of input neurons” and can identify which inputs are significantly utilized by the network. Using the full advantage of PL, we extracted the influential solar wind parameters that disturb the magnetosphere under southward Interplanetary magnetic field (IMF) conditions. The input parameters of PL were the three components of the IMF (Bx, By, Bz), solar wind flow speed (Vx), and proton number density (Np) in geocentric solar magnetospheric (GSM) coordinates obtained from the OMNI solar wind database between 1998 and 2019. Furthermore, we classified these input parameters into two groups (targets), depending on the Kp level: Kp = 6–9 (positive target) and Kp = 0 to 1 + (negative target). Negative target samples were randomly selected to ensure that numbers of positive and negative targets were equal. The PL results revealed that solar wind flow speed is an influential parameter for increasing Kp under southward IMF conditions, which was in good agreement with previous reports on the statistical relationship between the Kp index and solar wind velocity, and the Kp formulation based on the IMF and solar wind plasma parameters. Based on this new neural network, we aim to construct a more correct and parameter-dependent space weather forecasting model.Graphical

Proton and Helium Ion Acceleration at Magnetotail Plasma Jets

AbstractWe investigate two flow bursts in a series of Earthward bursty bulk flows (BBFs) observed by the Magnetospheric Multiscale spacecraft in Earth's magnetotail at (−24, 7, 4) RE in Geocentric Solar Magnetospheric coordinates. At the leading edges of the BBFs, we observe complex magnetic field structures. In particular, we focus on one BBF which contains large‐amplitude magnetic field fluctuations on the time scale of the proton gyroperiod, and another with a large scale dipolarization. For both events, the magnetic field structures are associated with flux increases of supra‐thermal ions with energies ≳100 keV. We observe that helium ions dominate the ion flux at energies ≳150 keV. We investigate the ion acceleration mechanism and its dependence on the mass and charge state of H+ and He2+ ions. We show that for both events, the ions with gyroradii smaller than the dawn‐dusk scale of the structure are accelerated by the ion bulk flow. For ions with larger gyroradii, the acceleration is likely due to a localized spatially limited electric field for the event with a large‐scale dipolarization. For the event with fluctuating magnetic field, the acceleration of ions with gyroradii comparable with the scale of the magnetic fluctuations can be explained by resonance acceleration.

Magnetic Field in Magnetosheath Jets: A Statistical Study of BZ Near the Magnetopause

AbstractMagnetosheath jets travel from the bow shock toward the magnetopause, and some of them eventually impact it. Jet impacts have recently been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018, https://doi.org/10.1002/2017gl076525) and Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357). In this study, we focus on the enhancing or suppressing effect jets could have on reconnection by locally altering the magnetic shear via their own magnetic fields. Using observations from the years 2008–2011 made by the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data, we statistically study for the first time within jets in the Geocentric Solar Magnetospheric coordinates. We find that opposite to the prevailing interplanetary magnetic field (IMF) is roughly as common in jets as in the non‐jet magnetosheath near the magnetopause, but these observations are distributed differently. 60–70% of jet intervals contain bursts of opposite polarity in comparison to around 40 of similar non‐jet intervals. The median duration of such a burst in jets is 10 s and strength is nT. We also investigate the prevalence of the type of strong nT pulses that Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357) linked to a substorm onset. In our data set, such pulses were observed in around 13% of jets. Our statistical results indicate that jets may have the potential to affect local magnetopause reconnection via their magnetic fields. Future studies are needed to determine whether such effects can be observed.

Numerical Algorithm for Detecting Ion Diffusion Regions in the Geomagnetic Tail With Applications to MMS Tail Season 1 May to 30 September 2017

AbstractWe present a numerical algorithm to identify ion diffusion regions (IDRs) in the geomagnetic tail and test its applicability. We use five criteria applied in three stages. (i) correlated reversals (within 90 s) of Vx and Bz (at least 2 nT about 0; geocentric solar magnetospheric coordinates); (ii) detection of Hall electric and magnetic field signatures; and (iii) strong (≥10 mV/m) electric fields. While no criterion alone is necessary and sufficient, the approach does provide a robust, if conservative, list of IDRs. We use data from the Magnetospheric Multiscale (MMS) mission spacecraft during a 5‐month period (1 May to 30 September 2017) of near‐tail orbits. We find 148 events satisfying Step 1, 37 satisfying Steps 1 and 2, and 17 satisfying all 3, of which 12 are confirmed as IDRs. All IDRs were within the X‐range [−24, −15] RE and the majority occurred during traversals of a tailward moving X‐line. Eleven of 12 IDRs were on the duskside despite approximately equal residence time in the plasma sheet (56.5% dusk vs. 43.5% dawn). MMS could identify signatures of four quadrants of the Hall B‐structure in three events and three quadrants in seven events. The events we report commonly display Vx reversals greater than 400 km/s in magnitude, normal magnetic field reversals often >10 nT in magnitude, maximum DC | |, which are often well in excess of the threshold for Stage 3. Our results are then compared with the set of IDRs identified by visual examination from Cluster in the years 2000–2005.

Origin of Solar Rotational Periodicity and Harmonics Identified in the Interplanetary Magnetic Field B_{z} Component Near the Earth During Solar Cycles 23 and 24

Periodic variations in solar-wind and geomagnetic parameters have long been recognized. In this work, we examine the periodic properties in the interplanetary magnetic field (IMF) near the Earth using satellite observations from 1996 to 2017. We pay particular attention to short-term periodicities (solar rotational period and its harmonics) of IMF B_{z} by distinguishing between the geocentric solar ecliptic (GSE) and the geocentric solar magnetospheric (GSM) coordinates. We find that, for nearly all of the years in Solar Cycles 23 and 24, IMF B_{z} exhibits periodic changes with a period of the solar rotation or its harmonics or both. We emphasize that these changes are far more pronounced in the GSM coordinates than in the GSE coordinates and during the northern hemisphere spring and fall seasons than the other two seasons. We attribute this result to an exquisite harmony between the Russell–McPherron effect and a well-defined IMF polarity that shows either a two- or four-sector structure for a long period of time.

Electron acceleration behind a wavy dipolarization front

In this paper, with the in-situ observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes we report a wavy dipolarization front (DF) event, where the DF has different magnetic structures and electron distributions at different $y$ positions in the Geocentric Solar Magnetospheric (GSM) coordinates. At $y \sim2.1R_{E}$ ( $R_{E}$ is the radius of Earth), the DF has a relatively simple structure, which is similar to that of a conventional DF. At $y \sim3.0R_{E}$ , the DF is revealed to have a multiple DF structure, where the plasma exhibits a vortex flow. Such a wavy DF could be the results of the interchange instability. The different structure of such a wavy DF at different sites has a great effect on electron acceleration. Fermi acceleration can occur at the site of the DF with a simple or multiple DF structure, while betatron acceleration as a local process has the contribution to energetic electrons only at the site of the DF with a simple structure.

Draping of strongly flow-aligned interplanetary magnetic field about the magnetopause

Many dynamic processes of the magnetosphere are directly driven by the solar wind and the occurrence of magnetic merging at the magnetopause. The location of magnetopause magnetic merging, or reconnection, is now fairly well understood when the interplanetary magnetic field (IMF) contains large By and Bz components in relation to the Bx component (in Geocentric Solar Magnetospheric (GSM) coordinates). However, when the IMF contains a large X-component (i.e., is closely flow-aligned), it is not yet well understood how the shocked IMF drapes about the magnetopause, and how this affects the occurrence and location of magnetic merging. In this initial study, we examine from observations how a nearly flow-aligned IMF drapes about the magnetopause. The results of this study are expected to be useful for comparisons with both analytic and global numerical models of the magnetosheath magnetic field.

Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity

Abstract. Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with BsVx2, where Bs is the southward component of the IMF and Vx is the solar wind velocity in the sun-earth direction. The am-BsVx2 relationship, however, depends on the range of Vx2: the am in higher ranges of Vx2 tends to be larger than am in lower ranges of Vx2 for the same value of BsVx2 for both equinoctial and solstitial epochs. Using the data sets of the same Vx2 range, it is shown that distribution of points in the am-BsVx2 diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each BsVx2 bin in solstitial epochs are closely consistent with those in equinoctial epochs, if Vx2 for each point at solstices are reduced to Vx2sin2 (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin2(ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect.

Evolution of remotely measured inner magnetospheric ion temperatures during a geomagnetic storm

[1] Previous studies have demonstrated that with the Medium Energy Neutral Atom instrument aboard the IMAGE spacecraft it is possible to remotely measure the ion temperature of the terrestrial magnetosphere during periods of strong geomagnetic activity. However, neutral atom imaging of the magnetosphere during quiet intervals is more difficult. In this work we show that by mapping neutral atom fluxes obtained over many days of observation to an equatorial plane fixed in geocentric solar magnetospheric coordinates, it is possible to construct neutral atom images of the quiet time magnetosphere. Enhanced neutral fluxes in the energy range of 1–70 keV/nucleon are observed in the quiet time premidnight region. A superposed-epoch analysis of multiple storm intervals also permits imaging of the ion temperature structure as a function of time through a geomagnetic storm. Using nearly 40 geomagnetic storms to produce average ion temperature maps as a function of storm phase, we find that there are significant differences between the spatial distribution of neutral fluxes and ion heating during the main phase of storms. Pronounced ion heating (up to 12 keV) is observed on the dayside during storm main phase from 5 to 8 Earth radii. During the early recovery phase, the ion temperature on the dayside drops to approximately 9 keV, and a colder region of approximately 6.5 keV persists near predawn. In the late recovery phase of the storm the ion temperature throughout the inner magnetosphere appears to relax to a nearly uniform 8 keV.

NGDC/GFZ candidate models for the 10th generation International Geomagnetic Reference Field

Abstract Following the call for candidates for the 10th generation IGRF, we produced and submitted three main field and three secular variation candidate models. The candidates are derived from parent models which use a standard quadratic parameterisation in time of the internal Gauss coefficients. External magnetospheric fields are represented by combined parameterisations in Solar Magnetic (SM) and in Geocentric Solar Magnetospheric (GSM) coordinates. Apart from the daily and annual variations caused by these external fields, the model also accounts for induction by Earth rotation in a non-axial external field. The uncertainties of our candidates are estimated by comparing independent models from CHAMP and Èrsted data. The root mean square errors of our main field candidates, for the internal field to spherical harmonic degree 13, are estimated to be less than 8 nT at the Earth’s surface. Our secular variation candidates are estimated to have root mean square uncertainties of 12 nT per year. A hind-cast analysis of the geomagnetic field for earlier epochs shows that our secular acceleration estimates from post-2000 satellite data are inconsistent with pre-2000 acceleration in the field. This could confirm earlier reports of a jerk around 2000.0, with a genuine change in the secular acceleration.

Effects of magnetic clouds on the occurrence of geomagnetic storms: The first 4 years of Wind

We investigate geomagnetic activity associated with magnetic clouds as well as some aspects of the clouds' magnetic field structure (e.g., magnetic field distributions of the clouds). Thirty‐four magnetic cloud events observed by Wind over the years 1995–1998 are investigated in this study. Magnetic clouds are a principal source of strong, long‐lasting, interplanetary, negative Bz fields (in solar magnetospheric coordinates) and hence are a major source of geomagnetic activity (Dst < −50 nT). The region just upstream of a cloud may be a significant source of southward fields instead of, or as well as, the cloud itself. We call this upstream region a “sheath” if it is bounded by a shock (or a pressure pulse) and the cloud. This study helps to identify the major sources (regions of southward Bz following shock passages or within the magnetic clouds) that are most dominant in the generation of geomagnetic storms. It is found that a geomagnetic storm can be induced by (1) a sheath, (2) the leading (i.e., front part) region of a cloud, (3) the trailing part of a cloud, and (4) both sheath and cloud regions. (Because of this complexity a storm with a multistep main phase can occur.) The related occurrence percentages of storms were 17.6% (six events), 44.1% (15 events), 5.9% (two events), and 20.6% (seven events), and the averaged storm intensities (minimum Dst) were −60, −85, −92, and −58 nT, respectively. For the remaining four events (11.8%), there were no storms. The occurrence timing of storm intensity is highly correlated with the occurrence timing of minimum Bz (maximum VBs or ϵ) for a magnetic cloud with the field rotating from southward to northward.

Structure of the magnetotail current sheet

An orbit tracing technique was used to generate current sheets for three magnetotail models. Groups of ions were followed to calculate the resulting cross‐tail current. Several groups then were combined to produce a current sheet. The goal is a model in which the ions and associated electrons carry the electric current distribution needed to generate the magnetic field B in which ion orbits were traced. The region −20 RE < x < −14 RE in geocentric solar magnetospheric coordinates was studied. Emphasis was placed on identifying the categories of ion orbits which contribute most to the cross‐tail current and on gaining physical insight into the manner by which the ions carry the observed current distribution. Ions that were trapped near z = 0, ions that magnetically mirrored throughout the current sheet, and ions that mirrored near the Earth all were needed. The current sheet structure was determined primarily by ion magnetization currents. Electrons of the observed energies carried relatively little cross‐tail current in these quiet time current sheets. Distribution functions were generated and integrated to evaluate fluid parameters. An earlier model in which B depended only on z produced a consistent current sheet, but it did not provide a realistic representation of the Earth's middle magnetotail. In the present study, B changed substantially in the x and z directions but only weakly in the y direction within our region of interest. Plasmas with three characteristic particle energies were used with each of the magnetic field models. A plasma was found for each model in which the density, average energy, cross‐tail current, and bulk flow velocity agreed well with satellite observations.

A statistical study of accelerated flow events at the dayside magnetopause

Accelerated plasma flows in the magnetopause current layer and low‐latitude boundary layer are examined using the ISEE 2 spacecraft. Accelerated flow of magnetosheath plasma inside the magnetopause is an indication that reconnection is occurring or has recently occurred. Over 180 inbound and outbound magnetopause crossings of the ISEE 2 spacecraft have been examined for such evidence of reconnection. Of those orbits for which a good determination could be made, a total of 58 statistically independent accelerated flow events were found where only one event is chosen per inbound or outbound magnetopause traversal. Upstream solar wind data are available for 42 of these 58 events. These 42 traversals are examined in relation to the upstream solar wind and magnetosheath parameters. The occurrence of accelerated flows is found to be dependent upon the direction of the upstream interplanetary magnetic field and the local magnetic shear across the magnetopause. The occurrence rate of accelerated flows is greatest when the interplanetary magnetic field has a southward component in solar magnetospheric coordinates and the local magnetic shear across the magnetopause is large. As the magnetosheath beta increases, the occurrence of accelerated flows decreases. Events in which the magnetic shear is small occur only when the beta is low. This study also provides a statistical estimate of the length and tilt of the merging line. The location and direction of the accelerated flows indicate that merging occurs most often near the subsolar point along a line of length ∼6 RE that tilts in response to the direction of the interplanetary magnetic field.

A large magnetosphere magnetic field database

A large magnetosphere magnetic field database compiled from the measurements of 11 Earth‐orbiting spacecraft during 1966‐1986 is described. The 11 contributing spacecraft (Explorer 33 and 35, IMP 4, 5, 6, 7 and 8, Heos 1 and 2, and ISEE 1 and 2) provide data between 4RE and 60RE, which have been edited in order to exclude data outside the magnetosphere. Currently, the database totals more than 79,000 records, which sum to a total of more than 1600 spacecraft days in the magnetosphere. Each of these records contains the average total field vector in geocentric solar magnetospheric coordinates, disturbance field vector (measured field minus Earth's internal field), time information, spacecraft position, AE, AL, Kp, and Dst geomagnetic indices, and associated solar wind and interplanetary magnetic field parameters when available. These data have constituted and will undoubtedly continue to constitute the standard database for magnetospheric modeling studies. They are also appropriate for statistical studies of the magnetic field and its dependence on solar wind quantities. Plans are to update this database in the future with data from additional spacecraft.

Field‐aligned currents in the Earth's magnetotail

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.

Influence of the Bx component of the interplanetary magnetic field on magnetopause reconnection

A first clear evidence of an effect of the Bx component (in Solar Magnetospheric Coordinates) of the Interplanetary Magnetic Field (IMF) on magnetopause reconnection has been obtained from polar ionospheric electric fields measured with balloons during an away sector of the IMF. This Bx ‐ effect is such that larger electric field values were measured when Bx was small, and vice versa, during intervals of time with Bz ≤ O, and while the transverse component of the IMF and the solar wind speed had approximately constant values. For those cases, changes of Bx form 2 to 8 gammas are found to account for as much as a factor of two decrease in the magnitude of the convection electric field at the polar cap. When Bz > O, there is also some indication that the measured dusk to dawn polar cap electric field (reversed convection) became larger when |Bx| was small and vice versa.

Dependence of the amount of open magnetic flux on the direction of the interplanetary magnetic field

The power generated by the solar wind—magnetosphere dynamo is proportional to the amount of the open magnetic flux Φ. It is difficult to use this fact in determining observationally the dependence of Φ on the orientation of the interplanetary magnetic field vector. It is shown that, for a simple vacuum superposition of the earth's dipole field and a uniform magnetic field, Φ is very closely proportional to sin θ/2) for a wide range of the intensity of the uniform field, where θ denotes the polar angle of the interplanetary magnetic field vector in the Y- Z plane of solar-magnetospheric coordinates.

Thinning of the near-earth (10∼15 RE) plasma sheet preceding the substorm expansion phase

The timing of the plasma-sheet thinning relative to the onset of the expansion phase of substorms is examined by the analysis of the OGO 5 electron (79 ± 23 keV) and proton (100∼150 keV) data with the aid of simultaneous magnetic field observations. It is found that the timing of the thinning is significantly dependent on the distance. At x 2 + y 2 ≲ 15 R E the thinning often starts before the onset, while at x 2 + y 2 ≳ 15 R E it tends to occur after the onset, where x and y refer to solar magnetospheric coordinates. The thinning that precedes the expansion-phase onset has been found to reduce the thickness to ∼1 R E , and further thinning may occur in a spatially limited region. Hence it is conceivable that the formation of the neutral line characterizing the substorm expansion phase is the consequence of the thinning of the plasma sheet in the near-Earth region.

ULF magnetic fluctuations in the plasma sheet as recorded by the Explorer 34 satellite

The Explorer 34 satellite was used as a platform for studying ultralow- frequency (ULF) magnetic waves from 0.01 to 0.2 Hz in the plasma sheet. The power spectral density was found to be proportional to the inverse square of the frequency in the plasma sheet and exhibited few, if any, significant spectral peaks. Further, the plasma sheet was characterized by power spectral density values that were higher than those of the high-latitude tail. The X/sub sm/, component where the subscript sm indicates solar magnetospheric coordinates, exhibited greater power spectral density values than the other two components. The most significant result, however, was that the power spectral density calculated for the frequency range 0.012-O.1 Hz in the plasma sheet increased from minimal values during quiet times to maximum levels during thc later stages of a substorin. It is shown that this result, in the frequency range studied, is probably inconsistent with the expectations of the substorm model of Hill and Dessler (1971), in which the plasma sheet was contained, during quiet times, by magnetic fluctuations.

Semiannual variation of geomagnetic activity

It is proposed that the semiannual variation in geomagnetic activity is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a magnetospheric system. Several simple models utilizing this effective modulation of the southward component of the interplanetary field are examined. One of these closely predicts the observed phase and amplitude of the semiannual variation. This model assumes that northward interplanetary fields are noninteracting and that the interaction with southward fields is ordered in solar magnetospheric coordinates. The prediction of the diurnal variation of the strength of the interaction at the magnetopause by this model, does not, however, match the diurnal variation of geomagnetic activity as derived from ground-based data.