scholarly journals Two Current Systems in the Preliminary Phase of Sudden Commencements in the Magnetosphere

2022 ◽  
Author(s):  
Shigeru Fujita ◽  
Takashi Tanaka
Keyword(s):  
Solar Wind ◽  
Current System ◽  
Alfven Wave ◽  
Lower Latitude ◽  
Geomagnetic Variation ◽  
Peak Displacement ◽  
Outer Magnetosphere ◽  
The Cross ◽  
Current Systems ◽  
Over Time

Abstract The geomagnetic variations of the preliminary impulse (PI) of the sudden commencement (SC) are known to show a time delay of the peak displacement and longer duration time in the higher latitudes in the pre-noon and post-noon sectors of the polar region. This peculiar behavior of the PI geomagnetic variation is associated with temporal deformation of the ionospheric PI field-aligned current (FAC) distribution into a crescent shape; its lower-latitude edge extends toward the anti-sunward direction, and its higher-latitude edge almost stays on the same longitude near noon. Numerical simulations revealed that the deformation of the FAC distribution is derived from different behaviors of the two PI current systems. The first current system consists of the FAC connected to the PI FAC in the lower latitude side of the ionosphere, the cross-magnetopause current, and the magnetosheath current (type L current system). The cross-magnetopause current is the inertia current generated in the acceleration front of the solar wind due to the sudden compression of the magnetosheath. Thus, the longitudinal speed of the type L current system in the ionosphere is the solar wind speed in the magnetosheath projected into the ionosphere. In contrast, the PI current system connected to the PI FAC at higher latitude (type H current system) consists of the upward/downward FAC in the pre-noon/post-noon sector, respectively, and dawn-to-dusk field-perpendicular current (FPC) along the dayside magnetopause. The dawn-to-dusk FPC moves to the higher latitudes in the outer magnetosphere over time. The FAC of the type H current system is converted from the FPC due to convergence of the return FPC heading toward the sunward direction in the outer magnetosphere; the return FPC is the inertia current driven by the magnetospheric plasma flow associated with compression of the magnetopause behind the front region of the accelerated solar wind. The acceleration front spreads concentrically from the subsolar point. Consequently, as the return FPC is converted to the FAC of the type H current system, it does not move much in the longitudinal direction over time because the dawn-to-dusk FPC of the type H current system moves to the higher latitudes. Therefore, the high-latitude edge of the PI current distribution in the ionosphere moves only slightly. Finally, we clarified that the FPC-FAC conversion of the type L current system mainly occurs in the region where the Alfvén speed starts to increase toward the Earth. A region with a steep gradient of the Alfvén speed like the plasmapause is not always necessary for conversion from the FPC to the FAC. We also suggest the possible field-aligned structure of the standing Alfvén wave that may occur in the PI phase.

scholarly journals Effect of the solar wind conditions on the ionospheric equivalent current systems

2013 ◽  
Vol 31 (3) ◽  
pp. 489-501 ◽  
Author(s):  
J. J. Zhang ◽  
C. Wang ◽  
B. B. Tang ◽  
H. Li
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Wind Speed ◽  
Current System ◽  
Solar Wind Speed ◽  
Lower Latitude ◽  
Equivalent Current ◽  
Mhd Model ◽  
Current Systems ◽  
The Imf

Abstract. We employ a global magnetohydrodynamics (MHD) model, namely the PPMLR-MHD model, to investigate the effect of the solar wind conditions, such as the interplanetary magnetic field (IMF) clock angle, southward IMF magnitude and solar wind speed, on the average pattern of the ionospheric equivalent current systems (ECS). A new method to derive ECS from the MHD model is proposed and applied, which takes account of the oblique magnetic field line effects. The model results indicate that when the IMF is due northward, the ECS are very weak while the current over polar region is stronger than the lower latitude; when the IMF rotates southward, the two-cell current system dominates, the eastward electrojet on the afternoon sector and the westward electrojet on the dawn sector increase rapidly while the westward electrojet is stronger than the eastward electrojet. Under southward IMF, the intensity of the westward electrojet and eastward electrojet both increase with the increase of the southward IMF magnitude and solar wind speed, and the increase is very sharp for the westward electrojet. Furthermore, we compare the geomagnetic perturbations on the ground represented by the simulated average ECS with the observation-based statistical results under similar solar wind conditions. It is found that the model results generally match with the observations, but the underestimation of the eastward equivalent current on the dusk sector is the main limitation of the present model.

scholarly journals Pseudo-field line resonances in ground Pc5 pulsation events

2005 ◽  
Vol 23 (2) ◽  
pp. 593-608 ◽  
Author(s):  
D. V. Sarafopoulos
Keyword(s):  
Solar Wind ◽  
Field Line ◽  
Large Scale ◽  
Research Effort ◽  
Current System ◽  
Oscillation Mode ◽  
Wind Pressure ◽  
Single Vortex ◽  
Solar Wind Pressure ◽  
Current Systems

Abstract. In this work we study four representative cases of Pc5 ground pulsation events with discrete and remarkably stable frequencies extended at least in a high-latitude range of ~20°; a feature that erroneously gives the impression for an oscillation mode with "one resonant field line". Additionally, the presented events show characteristic changes in polarization sense, for a meridian chain of stations from the IMAGE array, and maximize their amplitude at or close to the supposed resonant magnetic field shell, much like the typical FLR. Nevertheless, they are not authentic FLRs, but pseudo-FLRs, as they are called. These structures are produced by repetitive and tilted twin-vortex structures caused by magnetopause surface waves, which are probably imposed by solar wind pressure waves. The latter is confirmed with in-situ measurements obtained by the Cluster satellites, as well as the Geotail, Wind, ACE, and LANL 1994-084 satellites. This research effort is largely based on two recent works: first, Sarafopoulos (2004a) has observationally established that a solar wind pressure pulse (stepwise pressure variation) produces a twin-vortex (single vortex) current system over the ionosphere; second, Sarafopoulos (2004b) has studied ground events with characteristic dispersive latitude-dependent structures and showed that these are associated with twin-vortex ionosphere current systems. In this work, we show that each pseudo-FLR event is associated with successive and tilted large-scale twin-vortex current systems corresponding to a magnetopause surface wave with wavelength 10-20RE. We infer that between an authentic FLR, which is a spatially localized structure with an extent 0.5RE in the magnetospheric equatorial plane, and the magnetopause surface wavelength, there is a scale factor of 20-40. A chief observational finding, in this work, is that there are Pc5 ground pulsation events showing two gradual and latitude dependent phase-shifts of 180°, at the same time.

Impact of Solar Wind High Speed Streams on Ionospheric Current Systems

2020 ◽  
Author(s):  
Marcus Pedersen ◽  
Heikki Vanhamäki ◽  
Anita Aikio ◽  
Sebastian Käki ◽  
Ari Viljanen ◽  
...  
Keyword(s):  
Solar Wind ◽  
High Speed ◽  
Geomagnetic Storms ◽  
Current System ◽  
Global Scale ◽  
Ionospheric Current ◽  
Superposed Epoch Analysis ◽  
Interaction Regions ◽  
Current Systems ◽  
Dusk Sector

<p>High speed streams (HSS) and associated co-rotating interaction regions (CIR) in the solar wind are one of the major drivers of geomagnetic activity, especially during declining phases of sunspot cycles and near sunspot minima. We have identified 51 HSS/CIR driven geomagnetic storms that coincide with a Dst drop to less than -50nT during the period 2009-2018 and we investigate their impact on ionospheric current systems. Our approach is to study the evolution of the global scale current systems, i.e. the auroral electrojets and Region-1/2 field-aligned currents (FAC), with the SuperMAG magnetometers and AMPERE satellite data, respectively. The events are studied with a superposed epoch analysis centered at the storm onset to see the general behavior of the current system globally and in four different MLT sectors: noon, dusk, midnight and dawn. A minor enhancement of the integrated FAC was observed in the midnight, dawn and dusk sector 3 hours before the storm onset. The largest FAC and variability was observed in the dusk sector, and the integrated FAC maximum occurred in the middle of the storm main phase, 4 hours before the Dst minimum. This result will be compared to the evolution and behavior of the electrojet currents from superMAG. In the future a similar study will be conducted for ICME geomagnetic storms and compared to the HSS/CIR-related storms.</p>

scholarly journals Study of Solar Wind and Interplanetary Magnetic Field Features Associated with Geomagnetic Storms: The Cross Wavelet Approach

2021 ◽  
Author(s):  
Sujan Prasad Gautam ◽  
Ashok Silwal ◽  
Prakash Poudel ◽  
Monika Karki ◽  
Binod Adhikari ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Geomagnetic Storms ◽  
The Cross ◽  
Cross Wavelet

scholarly journals Azimuthal magnetic fields in Saturn’s magnetosphere: effects associated with plasma sub-corotation and the magnetopause-tail current system

2003 ◽  
Vol 21 (8) ◽  
pp. 1709-1722 ◽  
Author(s):  
E. J. Bunce ◽  
S. W. H. Cowley ◽  
J. A. Wild
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Magnetic Fields ◽  
Current System ◽  
Heavy Ion ◽  
Local Times ◽  
Dynamical Process ◽  
Tail Current ◽  
Field Lines ◽  
Inner Region

Abstract. We calculate the azimuthal magnetic fields expected to be present in Saturn’s magnetosphere associated with two physical effects, and compare them with the fields observed during the flybys of the two Voyager spacecraft. The first effect is associated with the magnetosphere-ionosphere coupling currents which result from the sub-corotation of the magnetospheric plasma. This is calculated from empirical models of the plasma flow and magnetic field based on Voyager data, with the effective Pedersen conductivity of Saturn’s ionosphere being treated as an essentially free parameter. This mechanism results in a ‘lagging’ field configuration at all local times. The second effect is due to the day-night asymmetric confinement of the magnetosphere by the solar wind (i.e. the magnetopause and tail current system), which we have estimated empirically by scaling a model of the Earth’s magnetosphere to Saturn. This effect produces ‘leading’ fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results show that the azimuthal fields observed in the inner regions can be reasonably well accounted for by plasma sub-corotation, given a value of the effective ionospheric Pedersen conductivity of ~ 1–2 mho. This statement applies to field lines mapping to the equator within ~ 8 RS (1 RS is taken to be 60 330 km) of the planet on the dayside inbound passes, where the plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet, and to field lines mapping to the equator within ~ 15 RS on the dawn side outbound passes. The contributions of the magnetopause-tail currents are estimated to be much smaller than the observed fields in these regions. If, however, we assume that the azimuthal fields observed in these regions are not due to sub-corotation but to some other process, then the above effective conductivities define an upper limit, such that values above ~ 2 mho can definitely be ruled out. Outside of this inner region the spacecraft observed both ‘lagging’ and ‘leading’ fields in the post-noon dayside magnetosphere during the inbound passes, with ‘leading’ fields being observed both adjacent to the magnetopause and in the ring current region, and ‘lagging’ fields being observed between. The observed ‘lagging’ fields are consistent in magnitude with the sub-corotation effect with an effective ionospheric conductivity of ~ 1–2 mho, while the ‘leading’ fields are considerably larger than those estimated for the magnetopause-tail currents, and appear to be indicative of the presence of another dynamical process. No ‘leading’ fields were observed outside the inner region on the dawn side outbound passes, with the azimuthal fields first falling below those expected for sub-corotation, before increasing, to exceed these values at radial distances beyond ~ 15–20 RS , where the effect of the magnetopause-tail currents becomes significant. As a by-product, our investigation also indicates that modification and scaling of terrestrial magnetic field models may represent a useful approach to modelling the three-dimensional magnetic field at Saturn.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Determination of the eddy turn-over time in the solar wind

2007 ◽  
Author(s):  
K. Bamert ◽  
R. Kallenbach ◽  
M. Hilchenbach ◽  
C. W. Smith
Keyword(s):  
Solar Wind ◽  
Turn Over ◽  
Over Time

scholarly journals Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

1999 ◽  
Vol 17 (4) ◽  
pp. 463-489 ◽  
Author(s):  
P. Prikryl ◽  
J. W. MacDougall ◽  
I. F. Grant ◽  
D. P. Steele ◽  
G. J. Sofko ◽  
...  
Keyword(s):  
Solar Wind ◽  
Electric Field ◽  
Alfven Waves ◽  
Alfven Wave ◽  
Alfvén Waves ◽  
Convection Cell ◽  
Convection Flow ◽  
Vhf Radar ◽  
Sky Imager ◽  
The Imf

Abstract. A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (Kp = 7- and IMF Bz < 0). The ionosondes measured electron densities of up to 9 × 1011 m-3 in the patch center, an increase above the density minimum between patches by a factor of \\sim4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF By > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF By) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.Key words. Ionosphere (polar ionosphere). Magneto- spheric physics (magnetosphere-ionosphere interactions; polar wind-magnetosphere interactions).

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