Intervals Of Interplanetary Magnetic Field Research Articles

Traveling Ionospheric Disturbances in the Vicinity of Storm-Enhanced Density at Midlatitudes.

This study provides first storm time observations of the westward‐propagating medium‐scale traveling ionospheric disturbances (MSTIDs), particularly, associated with characteristic subauroral storm time features, storm‐enhanced density (SED), subauroral polarization stream (SAPS), and enhanced thermospheric westward winds over the continental US. In the four recent (2017–2019) geomagnetic storm cases examined in this study (i.e., 2018‐08‐25/26, 2017‐09‐07/08, 2017‐05‐27/28, and 2016‐02‐02/03 with minimum SYM‐H index −206, −146, −142, and −58 nT, respectively), MSTIDs were observed from dusk‐to‐midnight local times predominately during the intervals of interplanetary magnetic field (IMF) Bz stably southward. Multiple wavefronts of the TIDs were elongated NW‐SE, 2°–3° longitude apart, and southwestward propagated at a range of zonal phase speeds between 100 and 300 m/s. These TIDs initiated in the northeastern US and intensified or developed in the central US with either the coincident SED structure (especially the SED basis region) or concurrent small electron density patches adjacent to the SED. Observations also indicate coincident intense storm time electric fields associated with the magnetosphere–ionosphere–thermosphere coupling electrodynamics at subauroral latitudes (such as SAPS) as well as enhanced thermospheric westward winds. We speculate that these electric fields trigger plasma instability (with large growth rates) and MSTIDs. These electrified MSTIDs propagated westward along with the background westward ion flow which resulted from the disturbance westward wind dynamo and/or SAPS.

Properties of Magnetic Field Fluctuations in Long-Lasting Radial IMF Events from Wind Observation

Long-lasting radial interplanetary magnetic field (IMF) intervals in which IMF points along the solar wind velocity for several hours have many interesting properties. We investigate the average parameters and the behavior of magnetic field fluctuations within 419 such radial intervals. The power spectral density (PSD) calculated over 1-h intervals of a radial IMF is compared with PSDs in adjacent regions prior to and after the radial IMF. We concentrate on (1) the power of IMF fluctuations, (2) the median slopes of PSDs in both inertial and kinetic ranges, (3) the proton temperature and its anisotropy, and (4) the occurrence rate of wavy structures and their polarization. We have shown that the fluctuation amplitude is low in the radial IMF intervals in both magnetohydrodynamic (MHD) and kinetic ranges, and the spectral power increases with the cone angle in the MHD range. We discuss this effect in the light of present knowledge on plasma turbulence and peculiarities of observations of magnetic field variations under the radial background magnetic field. We found that in the radial IMF events, the proton temperature is more isotropic, the occurrence rate of waves is higher, and the waves have no preferred polarization in the frequency range from 0.1 to 1 Hz. It suggests that the radial IMF structure leads to a different development of turbulence than the typical Parker-spiral structure.

Transpolar Arcs During a Prolonged Radial Interplanetary Magnetic Field Interval

AbstractTranspolar arcs (TPAs) are believed to predominantly occur under northward interplanetary magnetic field (IMF) conditions with their hemispheric asymmetry controlled by the Sun‐Earth (radial) component of the IMF. In this study, we present observations of TPAs that appear in both the northern and southern hemispheres even during a prolonged interval of radially oriented IMF. The Defense Meteorological Satellite Program (DMSP) F16 and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed TPAs on the dawnside polar cap in both hemispheres (one TPA structure in the southern hemisphere and two in the northern hemisphere) during an interval of nearly earthward‐oriented IMF on October 29, 2005. The southern hemisphere TPA and one of the northern hemisphere TPAs are associated with electron and ion precipitation and mostly sunward plasma flow (with shears) relative to their surroundings. Meanwhile, the other TPA in the northern hemisphere is associated with an electron‐only precipitation and antisunward flow relative to its surroundings. Our observations indicate the following: (a) the TPA formation is not limited to northward IMF conditions; (b) the TPAs can be located on both closed field lines rooted in the polar cap of both hemispheres and open field lines connected to the northward field lines draped over one hemisphere of the magnetopause. We believe that the TPAs presented here are the result of both indirect and direct processes of solar wind energy transfer to the high‐latitude ionosphere.

Can Enhanced Flux Loading by High‐Speed Jets Lead to a Substorm? Multipoint Detection of the Christmas Day Substorm Onset at 08:17 UT, 2015

AbstractThis paper describes highly interesting observations of the 25 December 2015 substorm onset at 08:17 UT during northward interplanetary magnetic field (IMF) while magnetosheath contained several intervals of negative Bz. A unique alignment of several spacecraft near the Earth‐Sun line together with magnetohydrodynamic simulations, ground‐based magnetometer, and auroral observations allow a comprehensive timing analysis of the events leading to substorm onset. Perplexingly, prior to substorm onset Geotail measured for 26 min positive IMF Bz just upstream of the bow shock, while simultaneously MMS spacecraft measured several intervals of strong negative Bz in the dayside magnetosheath. These strong pulses of negative Bz in the magnetosheath were associated with high dynamic pressure magnetosheath jets, likely created by foreshock transients during strongly radial IMF interval. Multipoint plasma and magnetic field measurements from ARTEMIS and THEMIS spacecraft were used to determine tail reconnection time at 8:14 and location at x=−33RE. Ground‐based observations of Pi2 pulsations and auroral brightening, with observations of a dipolarization front by THEMIS spacecraft, allowed determination of substorm onset to be at ≈08:17. All MMS spacecraft detected the same magnetosheath jet structure with Bz=−25 nT at ∼08:00 while IMF was northward. Based on DMSP observations and timing analysis we propose that these jets produced magnetopause reconnection leading to final, critical flux enhancement in the midtail region, which may have triggered reconnection 12–14 min later after jet observations.

The Effect of IMF Fluctuations on the Subsolar Magnetopause Position: A Study Using a Global MHD Model

AbstractDuring southward interplanetary magnetic field (IMF) conditions, the subsolar magnetopause moves inward even under steady solar wind pressure. This is generally referred to as “erosion.” Empirical and computational studies have been conducted to quantify the effect of magnetopause erosion; however, they generally use steady state IMF conditions. Solar wind conditions are not usually constant for very long, especially during such conditions as are present in high‐speed streams, which typically have large fluctuations in the IMF. These IMF fluctuations can occur on a range of times scales and amplitudes, resulting in a series of northward and southward IMF intervals imposed on the magnetosphere. We have simulated the effect of such IMF variations on the magnetopause position using a global magnetohydrodynamic model. We find that there are both time lag and hysteresis‐like effects on the motion of the magnetopause due to the variation in the magnitude of the southward IMF and that the instantaneous position of the magnetopause can be significantly different from what one would expect from steady state conditions.

Shape of the equatorial magnetopause affected by the radial interplanetary magnetic field

The ability of a prediction of the magnetopause location under various upstream conditions can be considered as a test of our understanding of the solar wind-magnetosphere interaction. The present magnetopause models are parametrized with the solar wind dynamic pressure and usually with the north-south interplanetary magnetic field (IMF) component. However, several studies pointed out an importance of the radial IMF component, but results of these studies are controversial up to now. The present study compares magnetopause observations by five THEMIS spacecraft during long lasting intervals of the radial IMF with two empirical magnetopause models. A comparison reveals that the magnetopause location is highly variable and that the average difference between the observed and predicted positions is ≈+0.7RE under this condition. The difference does not depend on the local times and other parameters, like the upstream pressure, IMF north-south component, or tilt angle of the Earth dipole. We conclude that our results strongly support the suggestion on a global expansion of the equatorial magnetopause during intervals of the radial IMF.

What Happens Before a Southward IMF Turning Reaches the Magnetopause?

AbstractPrevious observations have shown a ∼10–15 min time delay in the ionospheric response to solar wind directional discontinuities marked by either southward or northward interplanetary magnetic field (IMF) turnings. We have studied one southward IMF turning observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES in the dayside magnetosphere. Using a global MHD model, we have reproduced the magnetopause motion in this event. We find that the observed delay in the ground response can be completely explained by deceleration of the directional discontinuity in the subsolar magnetosheath. We show that the speed of the discontinuity significantly decreases in the vicinity of the magnetopause where the magnetic barrier formed during the previous northward IMF interval. The southward turning can reach the magnetopause only after complete disruption of the magnetic barrier. The disruption or dissipation occurs via magnetosheath reconnection, as confirmed by high‐speed jets in the magnetosheath. The magnetopause moves sunward as the directional discontinuity transits the magnetosheath. This sunward motion is followed by the earthward motion when the discontinuity strikes the magnetopause and magnetopause reconnection begins.

Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions

AbstractWe use the Time History of Events and Macroscale Interactions during Substorms data to investigate the magnetic field structure just outside the magnetopause and its time evolution for radial interplanetary magnetic field (IMF) events. When the magnetic field drapes around the magnetopause in the magnetosheath region, an asymmetric magnetic field orientation in different hemispheres is expected. Our two‐case study reveals some conflicts with the predicted draped field configuration in the Southern Hemisphere. The magnetosheath Bz component had a different sign depending on the upstream IMF Bx component's polarity at the beginning of the radial IMF intervals. With time, the observed Bz became northward in both cases with increasing positive values through the events. The increasing value of the Bz component may be explained by two possible mechanisms: by a change of the upstream IMF and by a reconnection between magnetosheath and magnetospheric field lines. Our study shows that both mechanisms contributed to the observed changes. Thus, there was a correlation between the change of the upstream IMF conditions and an increase in the magnetosheath northward magnetic field component. The observed formation of the boundary layer near the magnetopause proves that the reconnection process was ongoing at least for a part of the time. We suggest two possible reconnection scenarios: one near subsolar point and another tailward of the one cusp due to lobe reconnection. The asymmetry of reconnection locations causes rearrangement of the magnetic field structure near the magnetopause and turns the observed magnetosheath Bz component even further into positive values.

A method to predict magnetopause expansion in radial IMF events by MHD simulations

AbstractThis paper presents a method for taking into account changes of solar wind parameters in the foreshock using global MHD simulations. We simulate four events with very distant subsolar magnetopause crossings that occurred during quasi‐radial interplanetary magnetic field (IMF) intervals lasting from one to several hours. Using previous statistical results, we suggest that the density and velocity in the foreshock cavity decrease to ∼60% and ∼94% of the ambient solar wind values when the IMF cone angle falls below 50°. This diminishes the solar wind dynamic pressure to 53% and causes a corresponding magnetospheric expansion. We change the upstream solar wind parameters in a global MHD model to take these foreshock effects into account. We demonstrate that the modified model predicts magnetopause distances during radial IMF intervals close to those observed by THEMIS. The strong total pressure decrease in the data seems to be a local, rather than a global, phenomenon. Although the simulations with decreased solar wind pressure generally reproduce the observed total pressure in the magnetosheath well, the total pressure in the magnetosphere often agrees better with results for nonmodified boundary conditions. The last result reveals a limitation of our method: we changed the boundary conditions along the whole inflow boundary, although a more correct approach would be to vary parameters only in the foreshock. A model with the suggested global modification of the boundary conditions better predicts the location of part of the magnetopause behind the foreshock but may fail in predicting the rest of the magnetopause.

On the occurrence of magnetic reconnection equatorward of the cusps at the Earth's magnetopause during northward IMF conditions

AbstractMagnetic reconnection changes the topology of magnetic field lines. This process is most readily observable with in situ instrumentation at the Earth's magnetopause as it creates open magnetic field lines to allow energy and momentum flux to flow from the solar wind to the magnetosphere. Most models use the direction of the interplanetary magnetic field (IMF) to determine the location of these magnetopause entry points, known as reconnection lines. Dayside locations of magnetic reconnection equatorward of the cusps are generally found during sustained intervals of southward IMF, while high‐latitude region regions poleward of the cusps are observed for northward IMF conditions. In this study we discuss Double Star magnetopause crossings and a conjunction with a Polar cusp crossing during northward IMF conditions with a dominant IMF BY component. During all seven dayside magnetopause crossings, Double Star detected switching ion beams, a known signature for the presence of reconnection lines. In addition, Polar observed a cusp ion‐energy dispersion profile typical for a dayside equatorial reconnection line. Using the cutoff velocities for the precipitating and mirrored ion beams in the cusp, the distance to the reconnection site is calculated, and this distance is traced back to the magnetopause, to the vicinity of the Double Star satellite locations. Our analysis shows that, for this case, the predicted line of maximum magnetic shear also coincides with that dayside reconnection location.

Temporal evolutions of the solar wind conditions at 1 AU prior to the near‐Earth X lines in the tail: Superposed epoch analysis

AbstractUtilizing conjunction observations of the Geotail and ACE satellites from 1998 to 2005, we investigated the temporal evolutions of the solar wind conditions prior to the formation of X lines in the near‐Earth magnetotail. We first show the statistical properties of Bz, By, density, and velocity of the solar wind related to the 374 tail X line events. A superposed epoch analysis is performed to study the temporal evolutions of the solar wind conditions 5 h prior to the tail X lines. The solar wind conditions for tail X lines during southward interplanetary magnetic field (IMF) (SW‐IMF) and northward IMF (NW‐IMF) are analyzed. The main results are as follows: (1) For events classified as SW‐IMF, near‐Earth X line observations in the magnetosphere are preceded by ~2 h intervals of southward IMF; (2) for events classified as NW‐IMF, the northward IMF orientation preceding near‐Earth X line observations lasts ~ 40 min.

Steepening of waves at the duskside magnetopause

AbstractSurface waves at the magnetopause flanks typically feature steeper, i.e., more inclined leading (antisunward facing) than trailing (sunward facing) edges. This is expected for Kelvin‐Helmholtz instability (KHI) amplified waves. Very rarely, during northward interplanetary magnetic field (IMF) conditions, anomalous/inverse steepening has been observed. The small‐scale tetrahedral configuration of the Magnetospheric Multiscale spacecraft and their high time resolution measurements enable us to routinely ascertain magnetopause boundary inclinations during surface wave passage with high accuracy by four‐spacecraft timing analysis. At the dusk flank magnetopause, 77%/23% of the analyzed wave intervals exhibit regular/inverse steepening. Inverse steepening happens during northward IMF conditions, as previously reported and, in addition, during intervals of dominant equatorial IMF. Inverse steepening observed under the latter conditions may be due to the absence of KHI or due to instabilities arising from the alignment of flow and magnetic fields in the magnetosheath.

Quasi-continuous reconnection accompanied by FTEs during IMF Bz ≈ 0 nT observed by Double Star TC-1 at the dawnside magnetopause

During a one-hour interval of interplanetary magnetic field (IMF) Bz≈0nT, the equatorial spacecraft Double Star TC-1 encountered the dawn flank magnetopause many times at the magnetic local time (MLT) of about 08:00 and the latitude of about −27°. During each encounter, reconnection jets were observed with their velocities up to more than 500km/s, significantly higher than the background flow in the magnetosheath. The fast flows match the theoretical prediction of Alfvénic acceleration well. The medium temperature and density of ions in the boundary layer indicate the open magnetic field topology inside this layer. The mainly southward and tailward flows of the plasma jets alongside with the negative slopes of the Walén test indicate that the spacecraft was located south of the reconnection site, consistent with both anti-parallel and component reconnection models. The accelerated flows were observed lasting for about one hour, with some modulations by the oscillations of the magnetopause, but no reversals in the direction of Vz were found during the interval. The significantly enhanced flows in the boundary layer compared to the adjacent magnetosheath indicate that the reconnection was quasi-continuously active at the magnetopause northward of the spacecraft under such IMF conditions. At the same time, the bipolar signatures in BN with enhancements of the magnetic field indicate the occurrence of the Flux Transfer Events (FTEs). The observed reconnection was quasi-continuous, whereas the simultaneously accompanied FTEs were time-dependent under the IMF Bz≈0nT. For this event, however, it is not possible to identify whether the reconnection was anti-parallel or component because the TC-1 was far away from the reconnection site.

High‐resolution global magnetohydrodynamic simulation of bursty bulk flows

AbstractA high‐resolution global magnetohydrodynamic simulation is conducted with the Lyon‐Fedder‐Mobarry (LFM) model for idealized solar wind conditions. Within the simulation results high‐speed flows are seen throughout the magnetotail when the interplanetary magnetic field (IMF) is southward. Case study analysis of these flows shows that they have an enhancement in BZ and a decrease in density preceding a peak in the flow velocity. A careful examination of the structure within the magnetotail shows that these features are driven by bursts of magnetic reconnection. In addition to the case study, a superposed epoch analysis of flows occurring during a 90 min interval of southward IMF yields statistical properties that are in qualitative agreement with observational analysis of bursty bulk flows (BBFs). For the idealized simulation conditions, the most significant differences with the observational results are a broader velocity profile in time, which becomes narrower away from the center of the current sheet, and a larger density drop after flow passage. The peak BZ amplitude is larger than in observations and precedes the peak in the flow velocity. We conclude that the LFM simulations are reproducing the statistical features of BBFs and that they are driven by spatially and temporally localized reconnection events within the simulation domain.

Probabilistic forecasting analysis of geomagnetic indices for southward IMF events

Geomagnetic disturbances that drive space weather impacts such as ground-induced currents and radiation belt enhancements are usually driven by strong southward interplanetary magnetic field (IMF) intervals. However, current heliospheric models either do not predict or provide low-accuracy forecasts of IMF Bz. Here we examine the probability distribution function of geomagnetic activity indices for southward IMF intervals. We analyze the in situ plasma and magnetic field measurements long-duration large-amplitude southward IMF intervals (called Bs events). The statistical profiles of other solar wind and IMF parameters show significant differences during the periods 1 day before the Bs events for different solar wind transients (such as interplanetary coronal mass ejections and stream interaction regions). As is well known, we find that the solar wind speed is positively correlated with geomagnetic indices and that strong southward IMF is the key in storm triggering but not necessarily for substorms. We find that the solar wind density weakly affects geomagnetic field activity, but the response depends on the type of solar wind transient that includes the strong Bs events. We also find that magnetospheric ultralow-frequency waves are induced by both strong southward IMF and solar wind dynamic pressure disturbances. We suggest that strong Bs events could be predicted from the preceding characteristics of solar wind and IMF changes and that probabilistic forecasting of geomagnetic activity occurrence is potentially useful in space weather forecasting. We present preliminary analysis to demonstrate the out-of-sample ability to predict IMF Bs events with in situ solar wind data.

First results using TWINS‐derived ion temperature boundary conditions in CRCM

AbstractWe have integrated dynamic, spatiotemporally resolved ion temperature boundary conditions into the Comprehensive Ring Current Model (CRCM), which are based on 2‐D equatorial maps derived from the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) energetic neutral atom (ENA) data. The high‐speed stream‐driven event on 22 July 2009 is simulated and compared against an identical simulation using a statistically derived boundary condition model. ENA‐derived temperatures allow users to include event‐specific observations associated with a dynamic plasma sheet. This method also provides temperatures in the important region between geosynchronous orbit and the plasma sheet, a region which existing empirical models exclude. We find that the spatial and energy distributions of ring current flux and pressure have sensitive dependence on boundary conditions during this event. The coupling of boundary conditions to the time history of the convection field strength also plays an important role by throttling the influence of the boundary plasma on the inner magnetosphere. Simulated moments and spectra from our simulations are compared with remotely imaged ion temperatures from TWINS and also in situ energy spectra and temperature moments from Time History of Events and Macroscale Interactions during Substorms‐D. Storm time dusk‐dawn asymmetries consistent with observational data, such as Zhang et al. (2006), are reproduced well when CRCM is provided with the event‐specific boundary model. A hot localized structure observed by TWINS at geosynchronous midnight during a strong northward interplanetary magnetic field interval is also reproduced with this boundary model, whereas the empirical boundary model fails to yield this feature.

An MHD simulation study of the dynamics of the 8–9 March 2008 CIR‐/HSS‐driven geomagnetic storm

AbstractWe have carried out a global magnetohydrodynamic (MHD) simulation of a geomagnetic storm initiated by a corotating interaction region followed by a high‐speed solar wind (HSS) stream that occurred on 8–9 March 2008. The event began with the arrival of a corotating interaction region (CIR) at ~0720 UT on 8 March. The stream interface arrived at Earth at ~1830 UT on 8 March, and the arrival of a second density enhancement (a second CIR) at ~0140 UT on 9 March resulted in the main phase of the storm, with a peak Dst of −97 nT at 0600 UT on 9 March. Our MHD simulation of the event, spanning the interval of 0400 UT on 8 March to 0800 UT on 9 March, shows that the arrival of the first CIR changes the configuration of the magnetotail, and that after a strong substorm at ~1230 UT on 8 March, the tail evolves into a churning state in which the magnetic topology and flow structure of the magnetotail are never steady. In addition, we find that increases in ring current energy density show a nearly one‐to‐one correspondence to periods of VxBz > 0 (southward interplanetary magnetic field (IMF)). More importantly, we find that the ring current energy density in the MHD simulation shows a nearly linear response to increases in solar wind dynamic pressure, but only for the northward IMF intervals during the initial phase of the event, from 0400 UT to 1800 UT on 8 March.

The source, statistical properties, and geoeffectiveness of long‐duration southward interplanetary magnetic field intervals

AbstractGeomagnetic activity is strongly controlled by solar wind and interplanetary magnetic field (IMF) conditions, especially the southward component of IMF (IMF Bs). We analyze the statistical properties of IMF Bs at 1 AU using in situ observations for more than a solar cycle (1995–2010). IMF Bs events are defined as continuous IMF Bs intervals with varying thresholds of Bs magnitude and duration and categorized by different solar wind structures, such as magnetic cloud (MC), interplanetary small‐scale magnetic flux rope, interplanetary coronal mass ejection without MC signature (ejecta), stream interacting region, and Shock, as well as events unrelated with well‐defined solar wind structures. The statistical properties of IMF Bs events and their geoeffectiveness are investigated in detail based on satellite and ground measurements. We find that the integrated duration and number of Bs events follow the sunspot number when Bz < −5 nT. We also find that in extreme Bs events (t> 6 h, Bz < −10 nT), a majority (53%) are related to MC and 10% are related with ejecta, but nearly a quarter are not associated with any well‐defined solar wind structure. We find different geomagnetic responses for Bs events with comparable duration and magnitude depending on what type of solar wind structures they are associated with. We also find that great Bs events (t> 3 h, Bz < −10 nT) do not always trigger magnetic storms.

The source, statistical properties and geoeffectiveness of long-duration southward interplanetary magnetic field intervals

The source, statistical properties and geoeffectiveness of long-duration southward interplanetary magnetic field intervals

Sources of electron pitch angle anisotropy in the magnetotail plasma sheet

We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 RE from the Earth, using data from the Plasma Electron and Current Experiment (PEACE) instrument. We limit our survey to those pitch angle distributions measured when the interplanetary magnetic field (IMF) had been steadily northward or steadily southward for the previous 3 h. We find that, at sub‐keV energies, the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energy flux of electrons in the (anti‐) field‐aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked to the large‐scale field‐aligned current systems that couple the magnetosphere and ionosphere.