Magnetosphere-ionosphere Coupling Research Articles

On the plasma sheet dependence on solar wind and substorms and its role in magnetosphere-ionosphere coupling

Recently, it was argued that Hall conductivity and peak intensity of equivalent ionospheric currents are sensitive to the amount of field-aligned acceleration of plasma sheet (PS) electrons, which in turn depends on the plasma sheet parameters T e and N e (electron temperature and density) proportionally to the quantity eTN = (T e)1/2/N e. Here we extend these studies using data from six tail seasons of THEMIS observations to show statistically that the behavior of these PS electron parameters, measured in the middle of the nightside plasma sheet at ~10 RE distance, depends in a very different way on two basic processes: the solar wind state and substorms. We confirm previous work that slow/dense (fast/tenuous) solar wind provides cold/dense (hot/tenuous) plasma sheet conditions. However, we find that electron temperature and pressure parameters (T e and P e) behave differently from the proton ones (T p and P p), indicating a strong decoupling between temperature variations of auroral protons and electrons in the central plasma sheet (CPS): electrons are more sensitive to the substorm-related acceleration in the magnetotail than protons. Our superposed epoch study of plasma sheet parameter variations during substorms as well as our analysis of plasma acceleration at dipolarization fronts shows that during the substorm expansion phase a new (accelerated and plasma-depleted) population comes into the inner CPS with the flow bursts, showing an average increase of electron temperature and eTN parameter roughly by a factor of 2 above its background values for both cold/dense and hot/tenuous plasma sheet states. Preferential electron heating in the flow bursts is also statistically confirmed.

Occurrence characteristics and lowest speed limit of subauroral polarization stream (SAPS) observed by the SuperDARN Hokkaido East radar

We investigate the characteristics of the subauroral polarization stream (SAPS), with focus on the relationship between geomagnetic parameters and occurrence characteristics of SAPS. This study’s observations were made using the Super Dual Auroral Radar Network (SuperDARN) Hokkaido East radar, which can observe the Far East region of Russia and has been in operation since 2006. In particular, we identify the lowest limit of SAPS speed, which has not been discussed in previous literature, in order to examine the lowest threshold of electric field able to generate SAPS as a result of magnetosphere-ionosphere (M-I) coupling. In order to conduct a comprehensive investigation of SAPS occurrence characteristics, we analyzed events with wider ranges of velocity and magnetic latitude (MLAT) than those in previous studies. As a result of quantitative estimation, we found two categories of westward flows that were reasonably separated using a speed threshold of 150–200 m/s. For the faster flows above the speed threshold, there is a clear correlation between MLAT and SYM-H geomagnetic index, whereas for the slower flows, there is no such correlation. The faster flows are considered to be SAPS, whereas the slower flows are probably associated with mid-latitude F-region ionospheric irregularities not directly related to storms or substorms. This slowest limit of SAPS gives us a minimum electric field of 7.5–10 mV/m that generates SAPS. However, this field strength is not strong enough to cause frictional heating, which is generally considered to be a crucial mechanism for generating SAPS. This result suggests that frictional heating is not always necessary to generate SAPS.

A STATISTICAL SURVEY OF DYNAMIC PRESSURE PULSES IN THE SOLAR WIND BASED ONWINDOBSERVATIONS

Solar wind dynamic pressure pulse (DPP) structures, across which the dynamic pressure changes abruptly over timescales from a few seconds to several minutes, are often observed in the near-Earth space environment. The space weather effects of DPPs on the magnetosphere-ionosphere coupling system have been widely investigated in the last two decades. In this study, we perform a statistical survey on the properties of DPPs near 1 AU based on nearly 20 years of observations from the WIND spacecraft. It is found that only a tiny fraction of DPPs (around 4.2%) can be regarded as interplanetary shocks. For most DPPs, the total pressure (the sum of the thermal pressure and magnetic pressure) remains in equilibrium, but there also exists a small fraction of DPPs that are not pressure-balanced. The overwhelming majority of DPPs are associated with solar wind disturbances, including coronal mass ejection-related flows, corotating interaction regions, as well as complex ejecta. The annual variations of the averaged occurrence rate of DPPs are roughly in phase with the solar activity during solar cycle 23, and during the rising phase of solar cycle 24.

Global MHD simulation of magnetospheric response of preliminary impulse to large and sudden enhancement of the solar wind dynamic pressure

A sudden increase in the dynamic pressure of solar wind generates a prominent and transient change in ground-based magnetometer records worldwide, which is called a sudden commencement (SC). The magnetic field variation due to an SC at high latitudes shows a bipolar change, which consists of a preliminary impulse (PI) and main impulse (MI). The largest recorded SC had an amplitude of more than 200 nT with a spiky waveform at low latitudes, and the mechanism causing this super SC is unknown. Here, we investigate the cause of the super SC using a newly developed magnetosphere-ionosphere coupling simulation, which enables us to investigate the magnetospheric response to a large increase in the solar wind dynamic pressure. To simulate SCs, the dynamic pressure of the solar wind is increased to 2, 5, 10, and 16 larger than that under the stationary condition, and two different types of dynamic pressure increase are adopted by changing the solar wind density only or the solar wind speed only. It was found that the magnetic field variations of the PI and MI are several times larger and faster for a jump in the speed than for a jump in the density. It is inferred that a solar wind velocity of more than 2500 km/s in the downstream shock, which cannot be directly simulated in this study, would be consistent with the super SC.

Long-duration positive ionospheric storm during the December 2006 geomagnetic storm: Ionizing effect of forbidden electrons

The magnetosphere-ionosphere coupling at low latitudes was studied during the major geomagnetic storm on 14–16 December 2006. Data from NOAA/POES satellites were used to identify the enhancements of forbidden energetic electrons (FEE). Global Ionospheric Maps and COSMIC/FORMOSAT-3 radio occultation measurements were used for studying positive ionospheric storm phases. We found that long-lasting positive ionospheric storms were concomitant with FEE enhancements. We discussed relative contributions of the FEE ionizing effect as well as other general mechanisms to the positive ionospheric storm at different phases of the geomagnetic storm.

Decreased Sun-Earth energy-coupling efficiency starting from 2006

The geoeffect of extremely low solar (sunspot) activity starting from the last solar minimum is one of the major space science issues. The present study examines long-term changes of geomagnetic responses seen as Dst, Kp, and AL indices to the same solar wind conditions (density, velocity, magnetic field, and their products) using the NASA OMNI hourly values up to August 2014. Both decadal averages (1965 to 1974, 1975 to 1984, 1985 to 1994, 1995 to 2004 that approximately correspond to solar cycles #20 to 23, respectively, and from 2005) and annual averages indicate that the geomagnetic activity for a given solar wind condition, namely the Sun-Earth coupling efficiency, decreased quantitatively from around 2006 until now compared to the previous four decades. The decrease remains even after the EUV flux (using F10.7 index) is considered and is more obvious in the low-latitude geomagnetic disturbances (Dst) than in the high-latitude geomagnetic disturbances (AL). The results cannot be explained by existing explanations including the ionospheric conductivity effect in the magnetosphere-ionosphere coupling system.

ВОЗМУЩЕНИЕ ИОНОСФЕРЫ МОЩНЫМ НЕПРЕРЫВНЫМ ПРОХОДЯЩИМ РАДИОИЗЛУЧЕНИЕМ: РЕЗУЛЬТАТЫ ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ

Numerical simulations of disturbances in the electron temperature and density arising in the ionosphere from high-power continuous radio transmissions at various frequencies, power levels, and polarizations have been performed. It has been demonstrated that huge disturbances arise in the ionospheric D region electron temperature. In the E and F regions, the nocturnal electron temperature is also significantly disturbed. The electron density is disturbed much less than the electron temperature at all altitudes. Key words: ionosphere, high-power radio transmissions, numerical simulations, disturbances in the electron temperature and density Manuscript submitted 06.11.2014 Radio phys. radio astron. 2015, 20(1): 48-63 REFERECES 1. GUREVICH, A. V. and SHVARTSBURG, A. B., 1973. Nonlinear Theory of Radio waves Propagation in the Ionosphere. Moscow, Russia: Nauka Publ. (in Russian). 2. GUREVICH, A. V., 1978. Nonlinear Phenomena in the Ionosphere. New York, Heildelberg, Berlin: Springer-Verlag. https://doi.org/10.1007/978-3-642-87649-3 3. CHERNOGOR, L. F., 2014. Physics of High-Power Radio Emissions in Geospace: Monograph. Kharkiv, Ukraine: V. N. Karazin Kharkiv National Universine Publ. (in Russian). 4. GINZBURG, V. L. and GUREVICH, A. V., 1960. Nonlinear phenomena in a Plasma located in an alternating electromagnetic field. Usp. Fiz. Nauk. vol. 70, is. 2, pp. 201–246 (in Russian). DOI: https://doi.org/10.3367/UFNr.0070.196002a.0201 5. GINZBURG, V. L. and GUREVICH, A. V., 1960. Nonlinear phenomena in a Plasma located in an alternating electromagnetic field. Usp. Fiz. Nauk. vol. 70, is. 3, pp. 393–428 (in Russian). DOI: https://doi.org/10.3367/UFNr.0070.196003a.0393 6. GINZBURG, V. L., 1960. Propagation of Electromagnetic Waves in Plasmas. Moscow, Russia: Nauka Publ. (in Russian). 7. GUREVICH, A. V., 2007. Nonlinear effects in the ionosphere. Usp. Fiz. Nauk. vol. 177, is. 11, pp. 1145–1177 (in Russian). DOI: https://doi.org/10.1070/PU2007v050n11ABEH006212 8. GUREVICH , A. V. and SHLYUGER, I. S., 1975. Investigations of nonlinear phenomena with a powerful radio pulse in the lower ionosphere. Izv. Vysh. Uchebn. Zaved. Radiofizika. vol. 43, no. 9, pp. 1237–1260 (in Russian). 9. BRYUNELLI, B. E. and NAMGALADZE, A. A., 1987. Physics of the ionosphere. Moscow, Russia: Nauka Publ. (in Russian). 10. SCHUNK, R. W. and NAGY, A. F., 2000. Ionospheres: Physics, Plasma Physics, and Chemistry. – Cambridge: Cambridge University Press. https://doi.org/10.1017/CBO9780511551772 11. GOKOV, A. M., MARTYNRNKO, S. I., MISYURA, V. A., PIVEN', L. A., SOMOV, V. G., CHERNOGOR. L. F. and SHEMET, A. S., 1980. A study of natural and artificially disturbed by high-power radio emission ionosphere by high-frequency ionospheric sounding. In: The 2nd Conference on Polar Ionosphere and Magnetosphere-Ionosphere Coupling Proceedings. Norilsk, Russia, pp. 32–33 (in Russian). 12. Gokov, A. M., Martynrnko, S. I., Misyura, V. A., Piven', L. A., Somov, V. G., Fedorenko, Yu. P., CHERNOGOR. L. F. and SHEMET, A. S., 1982. Electron density profiles in the nighttime high-latitude lower ionosphere, artificially disturbed by high-power radio waves. Geomagnetizn i Aeronomiya. vol. 22, no. 15, pp. 748–752 (in Russian). 13. MARTYNRNKO, S. I., MISYURA, V. A., PIVEN', L. A., SOMOV, V. G., CHERNOGOR. L. F. and SHEMET, A. S., 1983. Disturbance of the inhomogeneous structure and regulary parameters in the middle and high-latitude lower ionosphere by continuous and periodic radiation. In: The Suzdal Symposium on Effects of high-power artificial radio emission on Earth ionosphere u1047 Proceedings. Sept 1983, Suzdal, Russia, pp. 94–95 (in Russian). 14. CHERNOGOR, L. F., 1985. Disturbance of the inhomogeneous structure in the lower ionosphere as a result of powerful radio emission. Radiophysics and Quantum Electronics. vol. 28, no. 1, pp. 12–20. DOI: https://doi.org/10.1007/BF01037452 15. CHERNOGOR, L. F., 1991. Diagnostics of Lower Ionosphere with Powerful Nonstationary Radio-Frequency Radiation. In: The III Suzdal URSI Symposium on Modification of the Ionosphere by Powerful Radiowaves (ISIM – 3) Proceedings. Suzdal, Russia, pp. 167–168. 16. HOLT, O., BREKKE, A., HANSEN, T., KOPKA, H. and STUBBE, P., 1985. HF modification of the auroral D-region detected by a partial reflection experiment. J. Atmos. Terr. Phys. vol. 47, no. 6, pp. 537–545. DOI: https://doi.org/10.1016/0021-9169(85)90036-4 17. KERO, A., BOSINGER, T., POLLARI, P., TURUNEN, E. and REITVELD, M. 2000. First EISCAT measurement of electron-gas temperature in the artificially heated D-region ionosphere. Ann. Geophys. vol. 18, no. 9, pp. 1210–1215. DOI: https://doi.org/10.1007/s00585-000-1210-8

Auroral vortex street formed by the magnetosphere–ionosphere coupling instability

Abstract. By performing three-dimensional magnetohydrodynamic simulations including Alfvén eigenmode perturbations most unstable to the ionospheric feedback effects, we examined the auroral vortex street that often appears just before substorm onset. We found that an initially placed arc splits, intensifies, and rapidly deforms into a vortex street. We also found that there is a critical convection electric field for growth of the Alfvén eigenmodes. The vortex street is shown to be a consequence of coupling between the magnetospheric Alfvén waves carrying field-aligned currents and the ionospheric density waves driven by Pedersen/Hall currents.

On the coupled interactions between ring current intensity and high-latitude ionospheric electron density variations

Investigations on the magnetospheric–ionospheric processes form an important element of research in the understanding of the solar–terrestrial interactions. In this work, we have investigated the linkage between the ring current intensity and the high-latitude ionospheric plasma density variations during different geomagnetic conditions. The Global Positioning System (GPS) derived Vertical Total Electron Content (VTEC) during 2011–2013 over high- and low-latitude stations in both the hemispheres and the symmetric ring current index (SYM-H) have been used in this study. A cross-correlation analysis performed between the variations of these two parameters during a wide range of geomagnetic conditions reveal that there is a seasonal, latitudinal and hemispherical dependence in the interrelationship between SYM-H and VTEC. The best cross-correlation between SYM-H and VTEC over both the hemispheres is obtained during equinoctial months which can be attributed to the semiannual variation of the solar wind–magnetospheric–ionospheric coupling. Summer time VTEC over southern hemisphere exhibits a better correlation with SYM-H index in comparison to that of the northern hemisphere. These results have been explained in the light of relative contributions from seasonal and hemispherical variation that exists in the ionospheric plasma. The results are striking as the correlation is found between the variation in two independent processes occurring at widely separated regions in space, namely, the ring current intensity and the behavior of ionospheric densities at high-latitudes. Season-dependent high- and low-latitude coupling of the ionospheric VTEC is observed during the disturbed geomagnetic conditions.

The geospace response to variable inputs from the lower atmosphere: a review of the progress made by Task Group 4 of CAWSES-II

The advent of new satellite missions, ground-based instrumentation networks, and the development of whole atmosphere models over the past decade resulted in a paradigm shift in understanding the variability of geospace, that is, the region of the atmosphere between the stratosphere and several thousand kilometers above ground where atmosphere-ionosphere-magnetosphere interactions occur. It has now been realized that conditions in geospace are linked strongly to terrestrial weather and climate below, contradicting previous textbook knowledge that the space weather of Earth's near space environment is driven by energy injections at high latitudes connected with magnetosphere-ionosphere coupling and solar radiation variation at extreme ultraviolet wavelengths alone. The primary mechanism through which energy and momentum are transferred from the lower atmosphere is through the generation, propagation, and dissipation of atmospheric waves over a wide range of spatial and temporal scales including electrodynamic coupling through dynamo processes and plasma bubble seeding. The main task of Task Group 4 of SCOSTEP's CAWSES-II program, 2009 to 2013, was to study the geospace response to waves generated by meteorological events, their interaction with the mean flow, and their impact on the ionosphere and their relation to competing thermospheric disturbances generated by energy inputs from above, such as auroral processes at high latitudes. This paper reviews the progress made during the CAWSES-II time period, emphasizing the role of gravity waves, planetary waves and tides, and their ionospheric impacts. Specific campaign contributions from Task Group 4 are highlighted, and future research directions are discussed.

Vortices at the magnetic equator generated by hybrid Alfvén resonant waves

We performed three-dimensional magnetohydrodynamic simulations of shear Alfvén waves in a full field line system with magnetosphere-ionosphere coupling and plasma non-uniformities. Feedback instability of the Alfvén resonant modes showed various nonlinear features under the field line cavities: (i) a secondary flow shear instability occurs at the magnetic equator, (ii) trapping of the ionospheric Alfvén resonant modes facilitates deformation of field-aligned current structures, and (iii) hybrid Alfvén resonant modes grow to cause vortices and magnetic oscillations around the magnetic equator. Essential features in the initial brightening of auroral arc at substorm onsets could be explained by the dynamics of Alfvén resonant modes, which are the nature of the field line system responding to a background rapid change.

Challenges in Measuring External Currents Driven by the Solar Wind-Magnetosphere Interaction

In studying the Earth's geomagnetism, it has always been a challenge to separate magnetic fields from external currents originating from the ionosphere and magnetosphere. While the internal magnetic field changes very slowly in time scales of years and more, the ionospheric and magnetospheric current systems driven by the solar wind -magnetosphere interaction are very dynamic. They are intimately controlled by the ionospheric electrodynamics and ionospheremagnetosphere coupling. Single spacecraft observations are not able to separate their spatial and temporal variations, and thus to accurately describe their configurations. To characterize and understand the external currents, satellite observations require both good spatial and temporal resolutions. This paper reviews our observations of the external currents from two recent LEO satellite missions: Space Technology 5 (ST-5), NASA's first three-satellite constellation mission in LEO polar orbit, and Communications/Navigation Outage Forecasting System (C/NOFS), an equatorial satellite developed by US Air Force Research Laboratory. We present recommendations for future geomagnetism missions based on these observations.

Aspects of magnetosphere–ionosphere coupling in sawtooth substorms: a case study

Abstract. In a case study we report on repetitive substorm activity during storm time which was excited during Earth passage of an interplanetary coronal mass ejection (ICME) on 18 August 2003. Applying a combination of magnetosphere and ground observations during a favourable multi-spacecraft configuration in the plasma sheet (GOES-10 at geostationary altitude) and in the tail lobes (Geotail and Cluster-1), we monitor the temporal–spatial evolution of basic elements of the substorm current system. Emphasis is placed on activations of the large-scale substorm current wedge (SCW), spanning the 21:00–03:00 MLT sector of the near-Earth plasma sheet (GOES-10 data during the interval 06:00–12:00 UT), and magnetic perturbations in the tail lobes in relation to ground observations of auroral electrojets and convection in the polar cap ionosphere. The joint ground–satellite observations are interpreted in terms of sequential intensifications and expansions of the outer and inner current loops of the SCW and their respective associations with the westward electrojet centred near midnight (24:00 MLT) and the eastward electrojet observed at 14:00–15:00 MLT. Combined magnetic field observations across the tail lobe from Cluster and Geotail allow us to make estimates of enhancements of the cross-polar-cap potential (CPCP) amounting to ≈ 30–60 kV (lower limits), corresponding to monotonic increases of the PCN index by 1.5 to 3 mV m−1 from inductive electric field coupling in the magnetosphere–ionosphere (M–I) system during the initial transient phase of the substorm expansion.

Isolating auroral FUV emission lines using compact, broadband instrumentation

Images of auroral emissions at far ultraviolet (FUV, 122–200nm) wavelengths are useful tools with which to study magnetospheric–ionospheric coupling, as the scattered sunlight background in this region is low, allowing both dayside and nightside auroras to be imaged simultaneously. The ratio of intensities between certain FUV emission lines or regions can be used to characterise the precipitating particles responsible for auroral emissions, and hence is a useful diagnostic of magnetospheric dynamics. Here, we describe how the addition of simple transmission filters to a compact broadband imager design allows far ultraviolet emission ratios to be deduced while also providing large-scale instantaneous images of the aurora. The low mass and volume of such an instrument would make it well-suited for both small satellite Earth-orbiting missions and larger outer planet missions from which it could be used to characterise the tenuous atmospheres observed at several moons, as well as studying the auroral emissions of the gas giants. We present a study to investigate the accuracy of a technique to allow emission line ratio retrieval, as applied to the OI 130.4nm and 135.6nm emissions at Ganymede. The ratio of these emissions provides information about the atmospheric composition, specifically the relative abundances of O and O2. Using modelled FUV spectra representative of Ganymede׳s atmosphere, based on observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS), we find that the accuracy of the retrieved ratios is a function of the magnitude of the ratio, with the best measurements corresponding to a ratio of ~1.3.

Windsock memory COnditioned RAM (CO‐RAM) pressure effect: Forced reconnection in the Earth's magnetotail

AbstractMagnetic reconnection (MR) is a key physical concept explaining the addition of magnetic flux to the magnetotail and closed flux lines back‐motion to the dayside magnetosphere. This scenario elaborated by Dungey (1963) can explain many aspects of solar wind‐magnetosphere interaction processes, including substorms. However, neither the Dungey model nor its numerous modifications were able to explain fully the onset conditions for MR in the tail. In this paper, we introduce new onset conditions for forced MR in the tail. We call our scenario the “windsock memory conditioned ram pressure effect.” Our nonflux transfer‐associated forcing is introduced by a combination of the large‐scale windsock motions exhibiting memory effects and solar wind dynamic pressure actions on the nightside magnetopause during northward oriented interplanetary magnetic field (IMF). Using global MHD Grand Unified Magnetosphere Ionosphere Coupling Simulation version 4 simulation results, upstream data from Wind, magnetosheath data from Cluster 1 and distant tail data from the two‐probe Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun mission, we show that the simultaneous occurrence of vertical windsock motions of the magnetotail and enhanced solar wind dynamic pressure introduces strong nightside disturbances, including enhanced electric fields and persistent vertical cross‐tail shear flows. These perturbations, associated with a stream interaction region in the solar wind, drive MR in the tail during episodes of northward oriented interplanetary magnetic field (IMF). We detect MR indirectly, observing plasmoids in the tail and ground‐based signatures of earthward moving fast flows. We also consider the application to solar system planets and close‐in exoplanets, where the proposed scenario can elucidate some new aspects of solar/stellar wind‐magnetosphere interactions.

Large-amplitude ULF waves at high latitudes

We present results from the statistical study of ULF waves detected by the fluxgate magnetometer in Gakona, Alaska during several experimental campaigns conducted at the High Frequency Active Auroral Research Program (HAARP) facility in years 2011–2013. We analyzed frequencies of ULF waves recorded during 26 strongly disturbed geomagnetic events (substorms) and compared them with frequencies of ULF waves detected during magnetically quiet times. Our analysis demonstrates that the frequency of the waves carrying most of the power in almost all these events is less than 1mHz. We also analyzed data from the ACE satellite, measuring parameters of the solar wind in the L1 Lagrangian point between Earth and Sun, and found that in several occasions there is a strong correlation between oscillations of the magnetic field in the solar wind and oscillations detected on the ground. We also found several cases when there is no correlation between signals detected on ACE and on the ground. This finding suggests that these frequencies correspond to the fundamental eigenfrequency of the coupled magnetosphere–ionosphere system, and the amplitude of these waves can reach significant magnitude when the system is driven by the external driver (for example, the solar wind) with this particular frequency. When the frequency of the driver does not match the frequency of the system, the waves still are observed, but their amplitudes are much smaller.

Storm-time meridional flows: a comparison of CINDI observations and model results

Abstract. During a large geomagnetic storm, the electric field from the polar ionosphere can expand far enough to affect the mid-latitude and equatorial electric fields. These changes in the equatorial zonal electric field, called the penetration field, will cause changes in the meridional ion flows that can be observed by radars and spacecraft. In general this E × B ion flow near the equator caused by the penetration field during undershielding conditions will be upward on the dayside and downward on the nightside of the Earth. Previous analysis of the equatorial meridional flows observed by CINDI instrument on the C/NOFS spacecraft during the 26 September 2011 storm showed that all of the response flows on the dayside were excess downward flows instead of the expected upward flows. These observed storm-time responses are compared to a prediction from a physics-based coupled model of thermosphere–ionosphere–inner-magnetosphere in an effort to explain these observations. The model results suggest that the equatorial downward flow could be attributed to a combined effect of the overshielding and disturbance dynamo processes. However, some discrepancy between the model and observation indicates a need for improving our understanding of how sensitive the equatorial electric field is to various model input parameters that describe the magnetosphere–ionosphere coupling processes.

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

Abstract. We study substorms from two perspectives, i.e., magnetosphere–ionosphere coupling across the auroral oval at dusk and at midnight magnetic local times. By this approach we monitor the activations/expansions of basic elements of the substorm current system (Bostrøm type I centered at midnight and Bostrøm type II maximizing at dawn and dusk) during the evolution of the substorm activity. Emphasis is placed on the R1 and R2 types of field-aligned current (FAC) coupling across the Harang reversal at dusk. We distinguish between two distinct activity levels in the substorm expansion phase, i.e., an initial transient phase and a persistent phase. These activities/phases are discussed in relation to polar cap convection which is continuously monitored by the polar cap north (PCN) index. The substorm activity we selected occurred during a long interval of continuously strong solar wind forcing at the interplanetary coronal mass ejection passage on 18 August 2003. The advantage of our scientific approach lies in the combination of (i) continuous ground observations of the ionospheric signatures within wide latitude ranges across the auroral oval at dusk and midnight by meridian chain magnetometer data, (ii) "snapshot" satellite (DMSP F13) observations of FAC/precipitation/ion drift profiles, and (iii) observations of current disruption/near-Earth magnetic field dipolarizations at geostationary altitude. Under the prevailing fortunate circumstances we are able to discriminate between the roles of the dayside and nightside sources of polar cap convection. For the nightside source we distinguish between the roles of inductive and potential electric fields in the two substages of the substorm expansion phase. According to our estimates the observed dipolarization rate (δ Bz/δt) and the inferred large spatial scales (in radial and azimuthal dimensions) of the dipolarization process in these strong substorm expansions may lead to 50–100 kV enhancements of the cross-polar-cap potential due to inductive electric field coupling.

M-I coupling scales and energy dumping

The paper reviews three magnetosphere-ionosphere coupling scales characterizing (1) inverted-V auroral arcs, (2) so-called Alfvenic arcs, and (3) dense plasma clouds artificially injected into the magnetosphere. The three scale-breaking processes are different but follow all from the principle of perfect matching of the wave impedance of the energy and momentum carrier with the effective resistance of the energy dump. As a consequence, wave reflections from the ionosphere are absent or present only in a short initial phase and lead to quasi-stationary wave fields. In inverted-V auroral arcs the energy conversion occurs in the low magnetosphere by postacceleration of the current-carrying hot electrons. For the Alfvenic arc the energy is transferred first to dispersive Alfven waves which are then damped in the topside ionosphere by accelerating electrons along B and ions transversely to B. Barium plasma clouds break into narrow striations mapping to ionospheric scales that experience a strongly reduced Pedersen conductivity and thus achieve the perfect matching. It is also argued that the scale ΣP/K, derived from electrostatic mapping, is not suited to describe M-I coupling.

Ionosphere-magnetosphere coupling and field-aligned currents

It is presented in this paper a review of one of several interactions between the magnetosphere and the ionosphere through the field-aligned currents (FACs). Some characteristics and physical implications of the currents flowing in a plane perpendicular to the magnetic field at high latitudes are discussed. The behavior of this system as an electric circuit is explained, where momentum and energy are transferred via Poynting flux from the magnetosphere into the ionosphere.