Southward Interplanetary Magnetic Field Bz Research Articles

Effects of solar wind ultralow‐frequency fluctuations on plasma sheet electron temperature: Regression analysis with support vector machine

AbstractTo investigate whether ultralow‐frequency (ULF) fluctuations from 0.5 to 8.3 mHz in the solar wind and interplanetary magnetic field (IMF) can affect the plasma sheet electron temperature (Te) near geosynchronous distances, we use a support vector regression machine technique to decouple the effects from different solar wind parameters and their ULF fluctuation power. Te in this region varies from ~0.1 to 10 keV with a median of 1.3 keV. We find that when the solar wind ULF power is weak, Te increases with increasing southward IMF Bz and solar wind speed, while it varies weakly with solar wind density. As the ULF power becomes stronger during weak IMF Bz (~0) or northward IMF, Te becomes significantly enhanced, by a factor of up to 10. We also find that mesoscale disturbances in a time scale of a few to tens of minutes as indicated by AE during substorm expansion and recovery phases are more enhanced when the ULF power is stronger. The effect of ULF powers may be explained by stronger inward radial diffusion resulting from stronger mesoscale disturbances under higher ULF powers, which can bring high‐energy plasma sheet electrons further toward geosynchronous distance. This effect of ULF powers is particularly important during weak southward IMF or northward IMF when convection electric drift is weak.

Role of IMF By in the prompt electric field disturbances over equatorial ionosphere during a space weather event

AbstractOn 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high‐latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF By are fundamentally important to understand the prompt penetration effects over low latitudes.

Relative contributions of momentum forcing and heating to high‐latitude lower thermospheric winds

AbstractWe discuss the significance of potential vorticity in the thermosphere and quantify the relative contributions of momentum forcing and heating to its total time derivative in the high‐latitude lower thermosphere during the southern hemisphere summertime for negative interplanetary magnetic field (IMF) Bz conditions on the basis of numerical simulations. A term analysis of the potential vorticity equation for weak or strong southward IMF (Bz = −2.0 nT or −10.0 nT) gives the following results: the ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF Bz = −2.0 nT are roughly 6:1, 4:1, and 2:1, respectively, indicating that the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, although the relative contribution of the momentum forcing weakens with descending altitude. The ratios of the momentum forcing term to the heating term at 142, 123, and 111 km altitudes for IMF Bz = −10.0 nT are roughly 3:1, 2:1, and 1:1, indicating that, at higher altitudes, the momentum forcing term makes the larger contribution to the total time derivative of the potential vorticity, but the relative contributions of momentum forcing and heating are comparable at lower altitudes. A comparison of the heating term and the momentum forcing term for IMF Bz = −2.0 nT and IMF Bz = −10.0 nT conditions indicates that the heating term increases more significantly than the momentum forcing term as IMF Bz becomes more negative.

Dynamic responses of the Earth's radiation belts during periods of solar wind dynamic pressure pulse based on normalized superposed epoch analysis

AbstractUsing the electron flux measurements obtained from five satellites (GOES 15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward interplanetary magnetic field (IMF) Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding postpulse recovery can extend to L ~ 3 and exceed the prepulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the preconditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

Modeling the storm time behavior of the magnetosonic waves using solar wind parameters

AbstractWe build empirical models of magnetosonic waves (also called as equatorial noise) both outside and inside the plasmapause, based on the Time History of Events and Macroscale Interactions during Substorms fast‐survey data during the period from 1 May 2010 to 30 November 2014, which utilize solar wind measurements as model inputs, i.e., interplanetary magnetic field (IMF) Bz, solar wind speed (VSW), and dynamic pressure (PSW). For model development, the time delay of magnetosonic wave amplitude relative to the solar wind parameters is first identified based on correlation analysis. We find that PSW is better correlated with the variation of wave amplitude outside the plasmapause than VSW, while the opposite is true inside the plasmapause. Among the solar wind parameters, the preceding southward IMF Bz with the time delay of 3–4 h and 4–5 h, corresponding to outside and inside the plasmapause, respectively, yields the best correlation with the variation of wave amplitude. In this study, we use the solar wind parameter‐based model to investigate magnetosonic wave activity during corotating interaction region‐driven storm. The results show that with increasing southward IMF Bz, wave activity is amplified mostly on the dayside both outside and inside the plasmapause and high amplitude is more shifted toward the prenoon sector, while increasing PSW, it is amplified only in the postnoon sector outside the plasmapause. Our statistical model can explain the observed amplitudes by 38% on average outside the plasmapause and 26% inside, and their model performance is generally higher than that of a model using AE index only as an input.

Duskside enhancement of equatorial zonal electric field response to convection electric fields during the St. Patrick's Day storm on 17 March 2015

AbstractThe equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand‐Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM‐A and SWARM‐C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.

Occurrence of equatorial spread F during intense geomagnetic storms

AbstractEquatorial spread F (ESF) has been observed in response to the prompt penetration of magnetospheric electric field to equatorial latitudes during intense (minimum Dst ≤ −100 nT; Bz ≤ −10 nT for at least 3 h) magnetic storms using global ion density plots of Defense Meteorological Satellite Program (DMSP) over nearly one solar cycle (1996–2005). Geostationary amplitude scintillation observations from Calcutta at VHF and L band for 1996–2005 and GPS amplitude scintillation measurements during 2004–2005 from the Indian Satellite Based Augmentation System Geostationary and GPS Navigation Outlay (GPS Aided GEO Augmented Navigation) network of stations all over India have been used to corroborate the DMSP observations. Subsequent to the time of southward interplanetary magnetic field Bz crossing −10 nT for an intense storm, it has been observed that within 4 h, ESF is generated at a longitude where the local time is dusk.

The effect of different solar wind parameters upon significant relativistic electron flux dropouts in the magnetosphere

AbstractSuperposed epoch analyses were performed on 193 significant relativistic electron flux dropout events, in order to study the roles of different solar wind parameters in driving the depletion of relativistic electrons, using ~16 years of data from the POES and GOES missions, and the OMNIWEB solar wind database. We find that the solar wind dynamic pressure and interplanetary magnetic field (IMF) Bz play key roles in causing the relativistic electron flux dropouts, but also that either large solar wind dynamic pressure or strong southward IMF Bz by itself is capable of producing the significant depletion of relativistic electrons. The relativistic electron flux dropouts occur not only when the magnetopause is compressed closer to the Earth but also when the magnetopause is located very far (> ~10 RE). Importantly, our results show that in addition to the large solar wind dynamic pressure, which pushes the magnetopause inward strongly and causes the electrons to escape from the magnetosphere, relativistic electrons can also be scattered into the loss cone and precipitate into the Earth's atmosphere during periods of strong southward IMF Bz, which preferentially provides a source of free energy for electromagnetic ion cyclotron (EMIC) wave excitation. This is supported by the fact that the strongest electron precipitation into the atmosphere is found in the dusk sector, where EMIC waves are typically observed in the high‐density plasmasphere or plume and cause efficient electron precipitation down to ~1 MeV.

Dependence of plasmaspheric hiss on solar wind parameters and geomagnetic activity and modeling of its global distribution

AbstractAccurate knowledge of the global distribution of plasmaspheric hiss is essential for the radiation belt modeling because it provides a direct link to understanding the radiation belt loss in the slot region. In this paper, we study the dependence of hiss activity on solar wind parameters and geomagnetic activity indices using Time History of Events and Macroscale Interactions during Substorms hiss measurements made from 1 July 2008 to 30 June 2012 based on a correlation analysis. We find that hiss amplitudes are well correlated with the preceding solar wind speed VSW, interplanetary magnetic field (IMF) BZ, and interplanetary electric field (IEF) EY with delay times of 5–6 h for VSW and 3–4 h for IMF BZ and IEF EY, while the best correlation with the geomagnetic indices, AE, Kp, and SYM‐H, occurs at a delay time of 2–3 h for AE and SYM‐H and 3–4 h for Kp. Of the solar wind parameters, the dawn‐to‐dusk component of IEF EY yields the best correlation with the variation of hiss wave. More interestingly, the global distribution of hiss waves shows a significant dependence on the VSW and IMF BZ: the most intense hiss region tends to occur at prenoon sector for a more southward IMF BZ, while the tendency is opposite with increasing VSW. This implies different origins of hiss activity. Also, we employ an artificial neural network technique to develop models of the global distribution of hiss amplitudes based on the solar wind parameters and geomagnetic indices. The solely solar wind parameter‐based model generally results in a higher correlation between the measured and modeled hiss amplitudes than any other models based on the geomagnetic indices. Finally, we use the solar wind parameter‐based model to investigate hiss activity during storm events by distinguishing between coronal mass ejection‐driven storms and corotating interaction region‐driven storms. The result shows that in spite of the differences in the behavior of solar wind parameters between the two storm groups, the different types of storms lead to the similar evolution of hiss waves in overall appearance even though the detailed behavior of hiss activations are different.

Statistics of ionospheric scintillation occurrence over European high latitudes

Rapid fluctuation in the amplitude and phase of transionospheric radio signals caused by small scale ionospheric plasma density irregularities is known as scintillation. Over the high latitudes, irregularities causing scintillation are associated with large scale plasma structures and scintillation occurrence is mainly enhanced during geomagnetic storms. This paper presents a statistical analysis of scintillation occurrence on GPS L1C/A signal at a high latitude station located in Bronnoysund (geographic latitude 65.5°N, geographic longitude 12.2°E; corrected geomagnetic (CGM) latitude 62.77°N), Norway, during the periods around the peaks of solar cycles 23 (2002–2003) and 24 (2011–2013). The analysis revealed that the scintillation occurrence at Bronnoysund during both the solar maximum periods maximises close to the midnight magnetic local time (MLT) sector. A higher occurrence of scintillation is observed on geomagnetically active days during both the solar maximum periods. The seasonal pattern of scintillation occurrence indicated peaks during the summer and equinoctial months. A comparison with the interplanetary magnetic field (IMF) components By and Bz showed an association of scintillation occurrence with the southward IMF Bz conditions.

Dependence of IMF By penetration into the neutral sheet on IMF Bz and geomagnetic activity

AbstractThe penetration of interplanetary magnetic field (IMF) By into the magnetosphere plays an important role in determining magnetospheric configuration and its dynamics. In this paper, using 9 years of Cluster data at the center of neutral sheet, we statistically study the relations of the penetration of IMF By in the neutral sheet (NS) with IMF Bz and the Kp index. The correlation coefficient between NS By and IMF By is enhanced during the periods of southward IMF Bz and large Kp indexes. The penetration efficiency of IMF By, which is defined as the slope of the linear fit of the points in the By‐IMF By space, is larger during southward IMF Bz than during northward IMF Bz. The penetration efficiency of IMF By also increases with increasing Kp index. Since the Kp index can be considered as an index of magnetospheric convection, this means that the penetration of IMF By into the magnetosphere is enhanced during the periods of strong magnetospheric convection. These results indicate that the IMF Bz and magnetospheric convection can influence the neutral sheet By and even magnetotail dynamics by changing the penetration of IMF By.

Solar wind control of auroral Alfvénic power generated in the magnetotail

The effects of solar wind driving conditions on the polar distribution of large‐scale, nondispersive Alfvénic Poynting flux at low altitude during steady magnetosphere convections are studied using three‐dimensional global simulations of the solar wind‐magnetosphere‐ionosphere interaction. Results from 18 test simulations driven by steady upstream solar wind (SW) and interplanetary magnetic field (IMF) conditions are used to investigate the relationship between SW/IMF driving and low‐altitude signatures of large‐scale Alfvénic Poynting flux. When the IMF is southward, the intensity of the Alfvénic Poynting flux increases, and the hemispheric integrated Alfvénic Poynting flux exhibits a linear relation with the SW electric field. When the IMF has a By component, the simulated hemispheric Alfvénic power does not fit to the same linear relation. During steady IMF By driving conditions, the low‐altitude regions with enhanced Alfvénic Poynting flux are magnetically connected with magnetospheric dynamo regions on both open and closed field lines. The physical origin of low‐altitude Alfvénic Poynting flux connecting to the closed field line region is similar with that during southward IMF Bz driving. However, the Alfvénic Poynting flux flowing from the open field line dynamo region may be related to the physical process on the magnetopause, and only shear‐mode waves are generated.

Simulated magnetopause losses and Van Allen Probe flux dropouts

AbstractThree radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon‐Fedder‐Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during ~ 14 h of strongly southward interplanetary magnetic field Bz beginning 8 October coincident with the inner boundary of outer zone depletion.

Relations of the energetic proton fluxes in the central plasma sheet with solar wind and geomagnetic activities

AbstractIn this paper, using 9 years of Cluster data, we statistically investigate the relations of central plasma sheet energetic proton fluxes, ~30 keV to 380 keV, with the solar wind parameters and geomagnetic indexes. The energetic proton fluxes increase with increasing solar wind dynamical pressure and solar wind speed. The energetic proton fluxes are more correlated with solar wind dynamical pressure than with solar wind speed. During northward interplanetary magnetic field (IMF) Bz, energetic proton fluxes are independent of northward IMF Bz, while during southward IMF Bz, energetic proton fluxes are highly correlated with southward IMF Bz and increase with increasing |IMF Bz|. The response time of energetic proton flux to southward IMF Bz is between 40 and 100 min. The energetic proton fluxes are correlated with plasma sheet ion temperature. The energetic proton fluxes increase with increasing indexes of Kp, AE, and |Dst|. Among the three geomagnetic indexes, the central plasma sheet energetic proton fluxes are most correlated with Kp index with the largest correlation coefficient being 0.82. The energetic proton fluxes are large during positive Dst index, suggesting that the sharp increase of solar wind dynamical pressure can enhance the plasma sheet energetic proton fluxes. The enhanced plasma sheet energetic proton fluxes may be important for geomagnetic storms and substorms since they can possibly directly become the source of ring current and substorm‐injected energetic particles without the need of additional acceleration process in the inner magnetosphere.

The double‐belt outer radiation belt during CME‐ and CIR‐driven geomagnetic storms

We have identified eight events with double‐belt structure in the outer radiation belt from 110 coronal mass ejection (CME) driven magnetic storms and 223 corotating interaction regions (CIR) driven storms during 1994 to 2003 based on the SAMPEX data sets. Among them, three cases are related to CME‐driven magnetic storms and five cases are related CIR‐driven storms. All double‐belt structure events in the outer radiation belt are found during the recovery phase of a magnetic storm for both CME‐ and CIR‐related events—they usually start to form within 3–4 days after the onset of the magnetic storm. The preconditions needed to form a double‐belt structure, for all the CME‐related events, are found to be high solar wind dynamic pressure (Pdy) and southward interplanetary magnetic field Bz; nevertheless, for the CIR‐related events, they are found to be associated with high‐speed stream with southward interplanetary magnetic field, which is enhanced by a suitably orientated By component.It is further found that the flux distributions of the double‐belt structure can be fitted well with a simply exponential decay function of L∗. Based on the radiation belt content index, the proportion of the total number of 1.5–6.0 MeV electrons inside the position of maximum fluxes to that outside the maximum fluxes keeps rising during the double‐belt period, which implies that the acceleration mainly occurs at regions inside the location of maximum fluxes. We suggest that the plasmapause and the strong wave‐particle interactions with VLF and ULF waves near it play an important role in the development of the double‐belt structures.

Empirically modeled global distribution of magnetospheric chorus amplitude using an artificial neural network

AbstractAccurate knowledge of the global distribution of magnetospheric chorus waves is essential for radiation belt modeling because it provides a direct link to understanding radiation belt losses and acceleration processes. In this paper, we report on newly developed models of the global distribution of chorus amplitudes based on in situ measurements of interplanetary magnetic field (IMF) and solar wind parameters as well as geomagnetic indices using an artificial neural network technique. We find that solar wind speed and IMFBZare the most influential parameters that affect the evolution of the magnetospheric chorus. The variations of chorus amplitudes in the outer (L≥ 7) and in the inner (5 ≤L< 7) regions, respectively, are well correlated with the variations of solar wind speed and IMFBZ. In addition, the solar wind parameter‐based chorus model generally results in a slightly higher correlation between measured and modeled chorus amplitudes than any other models including geomagnetic indices AE,Kp, andDst. The developed model shows that the chorus is amplified near the prenoon sector during the geomagnetically disturbed conditions. With increasing southward IMFBZthe location of peak chorus amplitude moves from the prenoon sector to the midnight sector, which is due to the enhanced electron injection near midnight. We also present a comparison of diffusive transport simulations for radiation belt electrons interacting with two newly developed chorus models, solar wind parameter‐based and geomagnetic index‐based chorus models. The comparison between two models shows that the modeling outside the plasmapause can affect the dynamic even inside the plasmasphere because the populations outside the plasmapause can act as seed population to radiation belt particles inside the plasmapause. One weakness of our chorus modeling is that it is trained during the early phase of solar cycle 24 where very few strong storms occurred. Therefore, our model might not be valid in reproducing the chorus activity under extremely disturbed conditions, which should be updated in the future once chorus measurements for such conditions become available.

The response of the dayside equatorial electrojet to step‐like changes of IMF BZ

The equatorial electrojet (EEJ) is driven by zonal electric fields, which are known to be well correlated with the interplanetary electric field and therefore with the interplanetary magnetic field (IMF) BZ component. In the present study, we investigate how the equatorial H magnetic component, and therefore the EEJ, responds to step‐like changes of IMF BZ. The reduction of southward IMF BZ (northward turning) and that of northward IMF BZ (southward turning) are examined separately. The result shows that for the northward turnings, the EEJ immediately starts to weaken with the accuracy of the estimates of the travel times of the IMF changes. The time constant of the response is much longer, and the equatorial H component decreases continuously by 40 nT for 30 min after the northward turnings. In contrast, the response of the EEJ to the southward turnings is far less clear in both magnitude and timing, and it does not depend on whether or not IMF BZ actually becomes southward. The difference in the EEJ response to the northward and southward turnings reflects at least partially the fact that the magnetosphere‐ionosphere system is more sensitive to IMF BZ when IMF is southward than northward. It is suggested that the electric field penetrates from the polar region to the dip equator through a global current system that connects the auroral electrojets and the EEJ, and the ionospheric conductance in the polar region may play an important role in the formation of such a current system.

Investigating the viscous interaction and its role in generating the ionospheric potential during the Whole Heliosphere Interval

The Whole Heliosphere Interval (WHI), comprising March 20–April 16, 2008 (DOY 80–107), is a single Carrington Rotation (2068) designated for intense study through observations and simulations. We used solar wind data from the WHI to run the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) and stand-alone Lyon–Fedder–Mobarry (LFM) models. The LFM model was also run with the WHI solar wind plasma parameters but with zero interplanetary magnetic field (IMF). With no IMF, we expect that the cross-polar cap potential (CPCP) is due entirely to the viscous interaction. Comparing the LFM runs with and without the IMF, we found that during strong driving with southward IMF Bz, the viscous potential could be a significant fraction of the total CPCP. During times of northward IMF Bz, the CPCP was generally lower than the CPCP value from the IMF=0 run. LFM tends to produce high polar cap potentials, but by using the Bruntz et al. (2012) viscous potential formula (ΦV=μn0.439V1.33, where μ=0.00431) and the IMF=0 LFM run, we calculated a scaling factor γ=1.54, which can be used to scale the LFM CPCP during the WHI down to realistic values. The Newell et al. (2008) viscous merging term can similarly be used to predict the viscous potential using the formula: ΦV=νn1/2V2, where the value ν=6.39×10−5 was also found using the zero IMF run. Both formulas were found to perform better when V (solar wind)=Vx, rather than Vtotal, yielding similar, accurate predictions of the LFM viscous potential, with R2>0.91 for both formulas. The γ factor was also used to scale down the LFM CPCP from the full solar wind run, with most of the resultant values matching the CPCP from the Weimer05 model well, even though γ was derived independent of the Weimer05 model or the full LFM data. We interpret this to be an indication that the conductivity model in LFM is producing values that are too low, thus elevating the CPCP values.

Bow shock and magnetopause contributions to the magnetospheric current system: Hints from the Cluster observations

We perform a statistical study of the surface current at the high‐latitude magnetopause (HLMP) and the bow shock under different southward interplanetary magnetic field (IMF) conditions, taking advantage of the crossing events of these discontinuities by the Cluster spacecraft. With an enhancement of southward IMF BZ, the surface current at HLMP reduces, while increasing at the bow shock. Since the amount of the magnetospheric current increases with the increase of southward IMF BZ, a synthesis analysis based on the Cluster observations suggests the bow shock and HLMP together contribute to the magnetospheric current system, and the bow shock would become an important current generator when the southward IMF BZ becoming large. This scenario accords with the previous global magnetohydrodynamic (MHD) simulations.

High Speed Stream Properties and Related Geomagnetic Activity During the Whole Heliosphere Interval (WHI): 20 March to 16 April 2008

We study the interplanetary features and concomitant geomagnetic activity of the two high-speed streams (HSSs) selected by the Whole Heliosphere Interval (WHI) campaign participants: 20 March to 16 April 2008 in Carrington rotation (CR) 2068. This interval was chosen to perform a comprehensive study of HSSs and their geoeffectiveness during this “deep” solar minimum. The two HSSs within the interval were characterized by fast solar-wind speeds (peak values > 600 km s−1) containing large-amplitude Alfvenic fluctuations, as is typical of HSSs during normal solar minima. However, the interplanetary magnetic field (IMF) magnitude [Bo] was exceptionally low (≈3 – 5 nT) during these HSSs, leading to lower than usual IMF Bz values. The first HSS (HSS1) had favorable IMF polarity for geomagnetic activity (negative during northern Spring). The average AE and Dst for the HSS1 proper (HSS1P) were + 258 nT and − 21 nT, respectively. The second HSS (HSS2) had a positive sector IMF polarity, one that is less favorable for geomagnetic activity. The AE and Dst index averages were + 188 nT and − 7 nT, both lower than corresponding numbers for the first event, as expected. The HSS1P geomagnetic activity is comparable to, and the HSS2P geomagnetic activity lower than, corresponding observations for the previous minimum (1996). Both events’ geomagnetic activities are lower than HSS events previously studied in the declining phase (in 2003). In general, Vsw was faster for the HSSs in 2008 compared to 1996. The southward IMF Bz was lower in the former. The product of these two parameters [Vsw and IMF Bz] comprises the solar-wind electric field, which is most directly associated with the energy input into the magnetosphere during the HSS intervals. Thus the combined effects led to the solar wind energy input in 2008 being slightly less than that in 1996. A detailed analysis of magnetic-field variances and Alfvenicity is performed to explore the characteristics of Alfven waves (a central element in the geoeffectiveness of HSSs) during the WHI. The Bz variances in the proto-CIR (PCIR) were ≈ 30 nT2 and < 10 nT2 in the high speed streams proper.