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• Formation of low magnetic field region (dent) in Earth's magnetosphere during intense storm. • Altitude and mirror points of charged particles get affected by magnetic field hump and dent. • The dent in magnetic field is formed closer to the Earth as strength of ring current increases. The development of the ring current through a westward circulation of energetic ions around the Earth in the low latitude region during geomagnetic storms has been known to us for the past several decades. The symmetric part of the ring current exists in the form of a ring around the Earth at a distance of ≈ 3–7 R e from the center of the Earth. The location and strength of the ring current is mainly controlled by the interplanetary solar wind conditions. The presence of such external current forms the additional mini magnetic dipole in the vicinity of the Earth's space, which eventually affects the geomagnetic field configuration. The magnetic field produced due to the ring current opposes the geomagnetic field inside and adds to the geomagnetic field outside the location of the ring current. This scenario has been modelled in this paper by using a circular loop of uniform current around the Earth to represent the ring current flow around the Earth. The aim of the present study is to understand the overall changes in the magnetic field and its influence on the radiation belt energetic (MeV range) electrons and protons during a geomagnetic storm. The model calculations suggest the formation of low and high magnetic field regions (here referred to as dent and hump) in the Earth's magnetosphere due to intense westward ring current. The size and position of these low and high magnetic field regions are controlled by the strength and location of the ring current. It is found that both the altitudinal reach and mirror point of the bouncing-drifting radiation belt energetic particles are significantly affected by the presence of the dent and the hump in the magnetic field driven by the ring current. The decrease in ground magnetic field of 1700 nT or more, which is similar to the geomagnetic field variation observed during the historic Carrington event, is possible with the peak ring current strength of 20–30 MA situated at a distance of 2 R e or less from the center of the Earth. [ABSTRACT FROM AUTHOR]

This study examines the variation of different energies linked with the Sun and the Earth’s magnetosphere-ionosphere systems for solar cycles (SCs) 22–24 for which the gradual decrease in the solar activity is noticed. Firstly, we investigated the variation of solar magnetic energy density (SMED) for SCs 21–24 and its relation to the solar activity. We observed distinct double peak structures in SMED for the past four SCs, 21–24. This feature is consistent with noticeable asymmetry in their two peaks. For SCs 22–24 a significant decrease is observed in the integrated SMED of each SC. This reduction is 37% from SCs 22 to 23 and 51% from SCs 23 to 24, which indicates substantial weakening of Sun’s magnetic field for SC 24. Also, the magnetic, kinetic, and thermal energy densities at the Earth’s bow-shock nose are found to be considerably low for the SC 24. We examined the solar wind Alfven speed, magnetosonic Mach number, solar wind-magnetosphere energy coupling parameter (ε), and the Chapman-Ferraro magnetopause distance (LCF) for the SCs 22–24. The estimated maximum stand-off magnetopause distance is larger for SC 24 (LCF ≤ 10.6 RE) as compared to SC 23 (LCF ≤ 10.2 RE) and SC 22 (LCF ≤ 9.8 RE). The solar wind Alfven speeds during SCs 22 and 23 are in the same range and do not exceed ≈73 km/s whereas, it is below 57 km/s for SC 24. A lower bound of solar wind magnetosonic Mach number for SC 24 is larger (M ≥ 6.9) as compared to SC 22 (M ≥ 5.9) and SC 23 (M ≥ 6). We noticed weakening in the energy coupling parameter for SC 24, which resulted in substantial (15%–38%) decrease in average strength of high latitude ionospheric (AE), low latitude magnetospheric (Dst) and equatorial ionospheric (EEJ) current systems in comparison with SC 23. Subsequently, a reduction of ≈30% is manifested in the high latitude Joule heating for SC 24. Overall this study indicates the significant step down in various energies at Sun, Earth’s bow-shock, and near Earth environment for current SC 24, which will have important implication on our Earth’s atmosphere-ionosphere-magnetosphere system.

There are several reports about the decrease of the magnetic field of our planet. Here, we present a detailed investigation of the geomagnetic field over past 200 years using magnetic field models (IGRF+gufm1) and ground magnetic field observations until 2020. There are two prominent regions of consistently decreasing geomagnetic field namely: the South Atlantic Anomaly (SAA) and the southern pole region. The SAA region with field decreasing at 30 nT/year drifting to the south-west, whereas, the southern pole region with field decreasing at the rate 13 nT/year steadily drifting to the north-west. The Earth's polar (66°–90°) magnetic field was invariably higher around the south pole as compared to the field around the north pole. However, it is found that over the past few decades (since 1950), the average magnetic field in the southern polar region has decreased to such an extent that presently it is weaker than the northern polar magnetic field. This tendency of cross-over in the magnitudes of average polar magnetic field strength is also evident in the ground magnetic field observations. On the ground at Hermanus (station in the SAA region), the rate of decrease of the magnetic field is found to be much higher (98 nT/year) during 1950–2020. Such a significant decrease in the magnetic field at Hermanus could be associated with the splitting of the SAA as it is situated close to the second minima in the SAA region. It is found that the magnetic field gradients around the center of the SAA become shallower as we move towards the higher altitudes. Also, the center of the SAA steadily moves in the north-east direction with increasing altitude. Though the weakening of the geomagnetic field is not an impulsive phenomenon but can invoke serious concerns to our life, satellites, and climate in near future. [ABSTRACT FROM AUTHOR]

• Weakened magnetic field is pushing radiation belt particles towards the Earth in SAA. • The average deepening rate of radiation belt particles is 4.3 km/year. • The deepening is increased by 434–590 km in last 120 years. • Deepening in the SAA region is depended on its initial longitude. • Deepening in the SAA region is independent of its equatorial pitch angle. The geomagnetic field has an unusual weak spot over South America and the South Atlantic Ocean, called South Atlantic Anomaly (SAA). We know that the magnetospheric particles trapped in the geomagnetic field move to the deeper altitudes over the SAA, resulting in the lower inner boundary of the radiation belt. However, over the past 400 years, the magnetic field in the SAA region has decreased consistently. In some recent studies, a rapid decrease of the magnetic field in the SAA region and the possibility of splitting of SAA have been reported. This weakened geomagnetic field has a bearing on the lower altitudinal reach (deepening) of the radiation belt particles. In this context, we performed simulations to examine and quantify the lowest altitudes encountered by the radiation belt particles over the SAA region. We found that particles of energies 100 keV-50 MeV, initially entering from an altitudes of 1–2 Earth radii are moving closer to the Earth at the rate of 3.8 - 4.7 km/year, which results in approximately 434 - 590 kms decrease in their altitude over the SAA region during 1900 to 2020. Furthermore, we noticed that the lowest altitude encountered by the particle in the SAA region is dependent on the initial longitude and independent of its initial local pitch angle corresponding to its entry in the Earth's magnetosphere. [ABSTRACT FROM AUTHOR]

We present a ground observation of modulations of strong electromagnetic ion cyclotron (EMIC) waves by short and long periodicities at Indian Antarctic station, Maitri. The signatures of these waves were evident in the magnetic field variations recorded by an induction coil magnetometer during the interval 4.7–7.2 UT on 17 September 2011, a moderately disturbed day. These waves were preceded by a gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising tone EMIC waves were observed in the Pc1 frequency band (∼0.5–0.9 Hz). The investigation of the periodicities of the observed wave spectrogram shows the presence of short (≈2.4 min) and long (≈39–69 min) periodicities. Our analysis shows that the short periodicities are associated with the Pc5 Ultra Low Frequency (ULF) waves generated by magnetic field line oscillations, while long periodicities might be associated with the ring current drifting ions. A new method, based on the cross‐correlation technique, is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in the range of 0.44–1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from the satellite observations of EMIC waves. We found that the higher sweep rates are associated with the stronger EMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that the theoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region is retained even during the propagation of these waves to the ground. Key Points: Short‐ and long‐period modulations of EMIC waves are observed at Indian Antarctic station MaitriShort periodicities are linked with Pc5 waves generated due to field line oscillations, and long periodicities with ring current drifting ionsHigher sweep rates are associated with stronger EMIC rising tone emissions on ground [ABSTRACT FROM AUTHOR]

We perform one-dimensional fluid simulation of ion acoustic (IA) solitons propagating parallel to the magnetic field in electron-ion plasmas by assuming a large system length. To model the initial density perturbations (IDP), we employ a KdV soliton type solution. Our simulation demonstrates that the generation mechanism of IA solitons depends on the wavelength of the IDP. The short wavelength IDP evolve into two oppositely propagating identical IA solitons, whereas the long wavelength IDP develop into two indistinguishable chains of multiple IA solitons through a wave breaking process. The wave breaking occurs close to the time when electrostatic energy exceeds half of the kinetic energy of the electron fluid. The wave breaking amplitude and time of its initiation are found to be dependent on characteristics of the IDP. The strength of the IDP controls the number of IA solitons in the solitary chains. The speed, width, and amplitude of IA solitons estimated during their stable propagation in the simulation are in good agreement with the nonlinear fluid theory. This fluid simulation is the first to confirm the validity of the general nonlinear fluid theory, which is widely used in the study of solitary waves in laboratory and space plasmas. [ABSTRACT FROM AUTHOR]