Bz Polarity Research Articles

Low Latitude Ionospheric TEC Oscillations Associated With Periodic Changes in IMF Bz Polarity

AbstractAn Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was recently deployed. Using ionospheric total electron content (TEC) from the two crossed Beidou geostationary satellite receiver chains of the network along 110°E and 23°N and Doppler velocity measurements from the Sanya (18.3°N, 109.6°E) portable digital ionosonde, we report first observations of low latitude TEC oscillations synchronized over a wide longitude range in East/Southeast Asia, which occur at nighttime, after the main phase of the geomagnetic storm on 20 April 2018. A comparison among TEC and Doppler velocity and interplanetary magnetic field (IMF) Bz component shows that the periodic TEC enhancements correlate with F region downward plasma drifts and IMF Bz southward turnings. The results suggest that the quasiperiodic southward turnings of IMF Bz could produce multiple short‐lived westward prompt penetration electric fields, which contribute to driving the nighttime low latitude TEC oscillations simultaneously over the wide longitude range.

Longitudinal variability of occurrence of ionospheric irregularities over the American, African and Indian regions during geomagnetic storms

This study presents the longitudinal variabilities of occurrence of ionospheric irregularities over the equatorial/low-latitude region of America, Africa and Indian sectors. Five major geomagnetic storms of the equinoctial months of the year 2012–2013 were analyzed for this study. Total Electron Content (TEC) data obtained from Global Navigation Satellite Systems (GNSS) over American, African and Indian longitudinal sectors were used to derive, the rate of change of TEC index (ROTIave), the ionospheric irregularities proxy. Longitudinal variability of occurrence of irregularities were discussed with respect to the local time when the maximum negative of SYM-H index occurs and polarity of IMF Bz in the dusk hours. Almost all of the storms under investigation inhibited the occurrence of irregularities in Indian sector. Out of five storms, two and three storm periods revealed inhibition in the generation of irregularities over the American and African sectors, respectively. Most of the generation of ionospheric irregularities observed in this study were related to post-midnight local time occurrence of the maximum negative excursion of SYM-H index. There were also cases where irregularities are observed when the minimum value of SYM-H occurred in the pre-midnight and daytime period. It appears that the local time at which minimum SYM-H occur is not a sufficient condition to explain the effect of storms on generation/inhibition of ionospheric irregularities. Significant longitudinal variability in the generation of irregularities were also observed over wide longitudinal ranges (South America, Africa and India) during 09 March 2012 and 17 March 2013 storm events. Difference in longitudinal variation of storm-time electric field were a suggested reason.

Magnetic Clouds: Solar Cycle Dependence, Sources, and Geomagnetic Impacts

Magnetic clouds (MCs) are transient magnetic structures giving the strongest southward magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry a southward magnetic field. The southward magnetic field is responsible for space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions for 1995 to 2017. 85% of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. MCs of the strongest magnetic magnitude and Bz south occur in the declining phase of the solar cycle. Bipolar MCs depend on the solar cycle in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency, but not in their polarity. MCs with the highest speeds, the largest total-B magnitudes, and sheath Bz south originate from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. Combinations of a south Bz in the sheath and south-first MCs in close succession have caused the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017 and now spans 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs that originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has a mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.

Interplanetary magnetic field properties and variability near Mercury's orbit

AbstractThe first extensive study of interplanetary magnetic field (IMF) characteristics and stability at Mercury is undertaken using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetometer data. Variations in IMF and solar wind conditions have a direct and rapid effect upon Mercury's highly dynamic magnetosphere; hence, understanding of the time scales over which these variations occur is crucial because they determine the duration of magnetospheric states. We characterize typical distributions of IMF field strength, clock angle, and cone angle throughout the duration of MESSENGER's mission. Clock and cone angle distributions collected during the first Earth year of the mission indicate that there was a significant north‐south asymmetry in the location of the heliospheric current sheet during this period. The stability of IMF magnitude, clock angle, cone angle, and IMF Bz polarity is quantified for the entire mission. Changes in IMF Bz polarity and magnitude are found to be less likely for higher initial field magnitudes. Stability in IMF conditions is also found to be higher at aphelion (heliocentric distance r ∼ 0.31 AU) than at perihelion (r ∼ 0.47 AU).

Antidipolarization fronts observed by ARTEMIS

AbstractNear‐Earth reconnection on closed plasma sheet field lines is thought to generate plasmoids. A plasmoid is usually described as a plasma sheet expansion into the lobe, encompassed by closed magnetic loops or the helical fields of a flux rope (in this paper we do not distinguish plasmoids from flux ropes; rather we use the term plasmoid generically). Recently, sharp, highly asymmetric north‐then‐south bipolar variations (with a larger southward portion) in the magnetic field BZ component have been noted in midtail (XGSM ~ −60 RE) plasmoids. These variations do not fit the classical plasmoid model but are mirror images of earthward moving dipolarization fronts (DFs), which show asymmetric south‐then‐north BZ bipolar variations with a larger northward portion. Using case and statistical studies from 3 years of Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) data (at XGSM ~ −60 RE), we show that magnetic and particle properties of these typically tailward moving fronts, which we refer to as “antidipolarization fronts” (ADFs), are very similar to those of classical, typically earthward moving DFs, except for their BZ polarity and flow direction. First, like DFs and plasmoids, ADFs are associated with auroral electrojet enhancements. Second, like DFs, ADFs exhibit a sharp density decrease, plasma pressure increase, magnetic pressure increase, and particle heating immediately following the sharp BZ change. Third, particle spectra indicate that, as with DFs, there are two distinctly different magnetically separated populations ahead of and behind ADFs. The energy spectrograms of plasmoids, however, indicate a single hot population at the plasmoid center. We conclude that midtail ADFs are likely products of fast reconnection, observed on the tailward side of the reconnection site, just as DFs are products of fast reconnection seen on the earthward side. ADFs are observed at ARTEMIS much less frequently (~10%) than typical plasmoids but twice as frequently as DFs at the same distance. We suggest that ADFs are protoplasmoids that emerge from near‐Earth reconnection and evolve quickly into plasmoids as they propagate down the tail.

The geo-effectiveness of interplanetary small-scale magnetic fluxropes

The geo-effectiveness of Interplanetary small-scale magnetic flux ropes (ISMFRs) are studied using multiple satellites (ACE, WIND, Geotail, Cluster, THEMIS, and geosynchronous spacecraft) and ground magnetometers. We identified 16 ISMFR events during 2007–2008 that had in situ observations of the near-Earth upstream solar wind in addition to observations from ACE and Wind at 1AU, and observations from multiple spacecraft in the inner magnetosphere. All the upstream solar wind (and in many cases magnetosheath) satellite observations showed very similar flux rope signatures indicating that the flux rope propagates from 1AU through the bow shock. Thirteen of the 16 events were associated with substorm activity while nine of them appeared to trigger isolated substorm onsets. Combined with earlier published databases of ISMFRs from 1995 to 2005, we also examined the geo-effectiveness using 1-min AE/AL indices. We found more than half of these events (73/141) were associated with substorms, while the rest were associated with quiet geomagnetic activity periods. Of the 73 substorm-related ISMFRs, 32 events had IMF Bz polarity signatures from south to north (SN), 31 from north to south (NS), and 10 were identified as By bipolar signature events. A superposed epoch analysis indicates that the timing of the substorm activity related to the ISMFRs is different between SN- and NS-events. Most of the ISMFRs associated with quiet geomagnetic activity were either By bipolar signature events or accompanied with complex Bz and By signatures. This study demonstrates that ISMFR with IMF Bz polarity signatures drive substorms, but not geomagnetic storms.

Penetration characteristics of the interplanetary electric field to the daytime equatorial ionosphere

Using 8 years of ionospheric drift measurements from the low‐latitude JULIA (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere) radar and the solar wind and interplanetary magnetic field data from the ACE (Advance Composition Explorer) satellite, we study the characteristics of the prompt penetration of electric fields to the equatorial ionosphere. A large database allowed us to bring out statistically significant characteristics of electric field penetration as a function of frequency. The coherence between the interplanetary electric field (IEF) and the equatorial electric field (EEF) peaks around a 2‐hour period with a maximum magnitude squared coherence of 0.6. The coherence is slightly higher (0.7) on magnetically active (Ap > 20) days. The cross‐phase spectra between the ACE and JULIA variations, after elimination of the propagation delay, have negligible values. Correspondingly, the time shift between IEF and EEF is less than 5 minutes at all periods. We also find that the penetration efficiency is highest during local noon, as compared with that of morning and evening hours. The coherence is lower for days with high solar flux values. We find that the penetration of electric fields into the equatorial ionosphere has no significant dependence on season and on the polarity of IMF Bz. We propose a transfer function between IEF and EEF, which was validated on synthetic as well as observed IEF data. The use of this transfer function decreases the misfit of a climatological model with the measured equatorial electric field by 27%.

Harmonics of Low Amplitude Anisotropic Wave Train Events in Cosmic Ray Intensity

The unusually low amplitude anisotropic wave train events (LAWEs) in cosmic ray intensity using the ground based Deep River neutron monitor data has been studied during the period 1991–1994. It has been observed that the amplitude of the diurnal anisotropy for LAWE events significantly remains quite low and statistically constant as compared to the quiet day annual average amplitude for majority of the events. The time of maximum of the diurnal anisotropy of LAWE significantly shifts towards earlier hours as compared to the co-rotational direction and remains in the direction of quiet day annual average anisotropy for majority of the events. On the other hand, the amplitude of the semi/tri-diurnal anisotropy remains statistically the same and high whereas, phase shift towards later hours as compared to the quiet day annual average values for majority of the LAWEs. The diurnal anisotropy vectors are found to shifts towards earlier hours for 50% of the events; whereas they are found to shifts towards later hours for rest of the events (50%) relative to the average vector for the entire period. It is also noted that the amplitude of these vectors are found to increase significantly with the shift of the diurnal anisotropy vectors towards later hours. The high-speed solar wind streams do not play a significant role in causing the LAWE events on short-term basis, however it may be responsible in causing these events on long-term basis (Mishra and Mishra 2007). Occurrence of LAWE is dominant, when the polarity of Bx and Bz remains positive and polarity of By remains negative, which is never been reported earlier. The amplitude of first harmonic shows good anti-correlation and direction of first and third harmonic shows nearly good anti-correlation with solar wind velocity, whereas the direction of second harmonic shows nearly good anti-correlation with interplanetary magnetic field strength.

Transient phenomena in cosmic ray intensity during extreme events

In the present work an analysis has been made of the extreme events occurring during July 2005. Specifically, a rather intense Forbush decrease was observed at different neutron monitors all over the world during 16 July 2005. An effort has been made to study the effect of this unusual event on cosmic ray intensity as well as various solar and interplanetary plasma parameters. It is noteworthy that during 11 to 18 July 2005 the solar activity ranged from low to very active. Especially low levels occurred on 11, 15, and 17 July whereas high levels took place on 14 and 16 July 2005. The Sun is observed to be active during 11 to 18 July 2005, the interplanetary magnetic field intensity lies within 15 nT, and solar wind velocity was limited to ∼500 kms-1. The geomagnetic activity during this period remains very quiet, the Kp index did not exceed 5, the disturbance storm time Dst index remains ∼-70 nT and no sudden storm commencement has been detected during this period. It is noted that for the majority of the hours, the north/south component of the interplanetary magnetic field, Bz, remains negative, and the cosmic ray intensity increases and shows good/high correlation with Bz, as the polarity of Bz tends to shift from negative to positive values, the intensity decreases and shows good/high anti-correlation with Bz. The cosmic ray intensity tends to decrease with increase of interplanetary magnetic field strength (B) and shows anti-correlation for the majority of the days.

Reversed flow events in the winter cusp ionosphere observed by the European Incoherent Scatter (EISCAT) Svalbard radar

High‐resolution fast azimuth sweeps by the European Incoherent Scatter (EISCAT) Svalbard radar provide an unparalleled opportunity to study small‐scale flow disturbances in the cusp ionosphere. Observations from 11 days of the winter cusp ionosphere of high‐resolution ion flow data have been analyzed. Transient channels of reversed plasma flow appear to be a regular feature of the cusp, and they were seen in 16% of 767 analyzed EISCAT Svalbard Radar (ESR) scans. We introduce a new descriptive term, reversed flow events (RFEs), for this class of events. RFEs are defined as longitudinally elongated segments of transiently enhanced ion flow in the direction opposite to the background flow. RFEs typically occurred near the cusp inflow region in association with enhancements in the polar cap convection observed by the Super Dual Auroral Radar Network (SuperDARN). Their lifetime was found to be ∼19 min on average. Their longitudinal dimension typically exceeded the ESR field of view (>400–600 km), and ranged from ∼50 to 250 km in latitude. The occurrence rate of RFEs appears independent of the BZ and BY component polarity of the interplanetary magnetic field (IMF), and RFEs occurred for clock angles between 40° and 240°. RFE ion flow was in 95% of the cases documented to oppose the magnetic tension force, and RFEs cannot be interpreted in terms of newly opened flux. RFEs formed one by one and never simultaneously in pairs. To explain these observations, we propose an asymmetric version of the Southwood (1987) twin cell flux transfer event model to account for significant IMF BY, in which only the poleward cell located on open field lines develops.

Effect of interplanetary transients on low amplitude anisotropic wave trains in cosmic rays

The occurrence of an unusual class of low amplitude anisotropic wave trains in the cosmic ray neutron intensity, which is distinctly different from the average diurnal variation as well as from other recognized types of low amplitude anisotropic wave trains are noted and the directional distribution in the interplanetary space determined. The major objective of this paper is to study the first three harmonics of low amplitude anisotropic wave trains of cosmic ray intensity over the period 1991–1994 for Deep River neutron monitoring station. The significant characteristic of these events is that the low amplitude wave trains shows a maximum intensity of diurnal component in a direction earlier than 1800 h/co-rotational direction. It is noticed that these events are not caused either by the high-speed solar wind streams or by the sources on the Sun responsible for producing these streams such as polar coronal holes. The direction of the tri-diurnal anisotropy shows a good negative correlation with Bz component of interplanetary magnetic field. The occurrence of low amplitude events is dominant for positive polarity of Bz. The Disturbance Storm Time index i.e., D st, remains consistently negative only throughout all the low amplitude wave train events, which is never been reported earlier.

Solar wind control of Earth's H+ and O+ outflow rates in the 15‐eV to 33‐keV energy range

Earth's high‐latitude outflow of H+ and O+ ions has been examined with the Toroidal Imaging Mass‐Angle Spectrograph instrument on the Polar satellite in the 15‐eV to 33‐keV energy range over an almost 3‐year period near solar minimum (1996–1998). This outflow is compared with solar wind plasma and interplanetary magnetic field (IMF) data from the Wind spacecraft, the latter having been time shifted to the subsolar magnetopause and averaged for 15 min prior to each sampling of Earth's magnetic field‐aligned ion flow densities. When the flow data are arranged according to the polarity of the IMF Bz (in GSM coordinates) and limited to times with Bz > 3 nT or Bz < −3 nT, the total rate of ion outflow is seen to be significantly enhanced with negative Bz, typically by factors of 2.5–3 for the O+ and 1.5–2 for the H+, more than previously reported from similar but less extensive comparisons. With either IMF Bz polarity the rate of ion outflow is well correlated with the solar wind energy flow density, especially well with the density of kinetic energy flow. The rate of ion outflow within the instrument's energy range is a strong function of the Polar satellite altitude, increasing almost threefold from perigee (R ∼ 2 RE) toward apogee (R ∼ 4–9 RE) for O+ ions, i.e., up to 1026 ions s−1 or more per hemisphere. The apogee enhancement may be still larger for the H+, but it is obscured by mantle flow of cusp origin solar H+. Ion mean energy also increases with altitude, leading to about a twentyfold increase in the O+ energy flow rate from Polar perigee to apogee altitude, reaching values of 20 GW or more per hemisphere. While the perigee outflow of H+ has little or no seasonal modulation, in terms of ions s−1 the O+ outflow rates at both altitudes do increase during local summer and so does the rate of cusp origin H+ flow near apogee. The latter rate, in fact, has very similar seasonal modulation as the O+ rates, suggesting that it has a significant influence on the O+ outflow.

Magnetospheric disturbance induced equatorial plasma bubble development and dynamics: A case study in Brazilian sector

Equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 August 1998 are investigated using the data collected by a multistation and multi‐instrument diagnostic network operated at equatorial and low latitude sites in Brazil, and auroral electrojet activity (AU/AL), IMF, and Dst indices. A magnetospheric disturbance onset in the morning of 26 August 1998 was initiated by a solar wind shock and associated IMF Bz polarity reversals and ssc that were soon followed by a succession of substorm‐like auroral electrojet (AE) intensifications and Dst development. An IMF Bz southward turning and associated AE intensifications in the Brazilian dusk sector produced intense prompt penetration eastward electric field that caused large F region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles, the latter event lasting the entire night, as observed by digital ionosondes at São Luís (2.33°S, 315.8°E, dip angle: −.5°) and Fortaleza (3.9°S, 321.55°W, dip angle: −9°) and an all‐sky imager, two scanning photometers, and a Digisonde at the low‐latitude site Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°). A notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours. The results show for the first time a relationship between the zonal drift velocities of optically observed large‐scale bubbles (tens to hundreds of kilometers) and that of the smaller scale (kilometer sizes) structures as observed by a digital ionosonde. The results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours. As an important finding, the results further show that significant contribution to the westward plasma bubble irregularity drift, normally attributed to disturbance dynamo effect, could arise from prompt penetration disturbance zonal electric field, in the course of a disturbance sequence lasting several hours. Such effect is attributed to Hall electric field arising from the primary disturbance zonal electric field, under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation, in the Brazilian longitude sector in agreement with recent evidences [Abdu et al., 1998b].

Solar wind density and velocity control of auroral brightness under normal interplanetary magnetic field conditions

In this study we determine relationships between auroral brightness and each of the solar wind density and velocity under normal interplanetary magnetic field (IMF) conditions using Polar Lyman‐Birge‐Hopfield long band (∼170.0 nm) images and Interplanetary Monitoring Platform 8 solar wind data. Since IMF and season are parameters that can affect the auroral brightness, the determination of the relationships is performed on the basis of the IMF Bz polarity and season to reduce some contamination from these two parameters. It is found that for southward IMF and the winter case the solar wind density and velocity increase the auroral brightness mainly on the nightside. For southward IMF and the summer case the density and velocity also increase the auroral brightness mainly on the nightside, but the increase is not as large as that for southward IMF and the winter case.

EISCAT Svalbard radar observations of ionospheric signatures of magnetopause reconnection during a changing IMF Bz polarity

Abstract. Observations by the EISCAT Svalbard radar are presented that show the response of the spatial structure of the ionosphere in the dayside cusp region to a rotational trend in the IMF clock angle. Over a period of one hour, the clock angle increased from about 45° to some 150°, moving the likely location of the magnetopause reconnection site from the high-latitude lobe to near the equatorial plane. Increased topside electron temperatures measured by the ESR identified footprints of the reconnection process. Temporal changes in the spatial distribution of the temperature reflected the change from lobe to equatorial reconnection. Discrete spatial enhancements in ion temperature were found resulting from ion-neutral frictional heating in the fast flows where it was likely that field lines were being convected from the reconnection locations. The corresponding electron density structuring is interpreted in terms of the particle precipitation, field-aligned currents and convection flows driven by the IMF.Key words. Ionosphere (ionosphere – magnetosphere interactions; plasma temperature and density; polar ionosphere)

Evidence of different magnetotail responses to small solar wind pressure pulses depending on IMF Bz polarity

The response of the magnetotail to small flux rope associated pressure pulses depends on the IMF Bz polarity. Clear and large amplitude magnetotail reverse polarity South‐then‐North traveling compression regions (SN TCRs) are observed when the pressure pulse is associated with the northward IMF interval of the flux rope. These tail signatures correspond to small‐scale (<10 minute duration) pulses of the solar wind dynamic pressure located in front of and within the small‐scale flux ropes. Pressure pulses associated with IMF Bz southward are either absent or give rise to smaller amplitude compressions and less systematic lobe Bz rotations. These results suggest that the orientation of the IMF Bz plays a critical role in modulating the magnetotail response to solar wind dynamic pressure pulses.

Ionospheric convection response to changes of interplanetary magnetic field Bz component during strong By component

It is well established that ionospheric convection patterns are strongly controlled by the interplanetary magnetic field (IMF). There has been an increased interest in ionospheric convection response to IMF changes. Some studies found that ionospheric flows change first near noon and later near the dawn‐dusk meridian plane, which is interpreted as propagation or expansion of newly generated convection cells in the cusp region. Other studies showed that the change in convection pattern in response to IMF reorientations is spatially fixed. In this paper, we investigate the ionospheric convection response to IMF Bz changes during strong IMF By. On March 23, 1995, Bx was small, By was strongly positive (7–11 nT), and the Bz polarity changed several times after 1300 UT. The dayside ionospheric convection is dominated by a large clockwise convection cell. The cell focus (the “eye” of the convection pattern) is located in the prenoon sector for northward Bz and in the postnoon sector for southward Bz. It is found that the cell focus shifts from the prenoon sector to the postnoon sector following a southward Bz turning and vice versa for a northward Bz turning. However, the motion of the convection cell, or the largest change in the convection pattern, is limited roughly to the region between the previous cell focus and the new cell focus. Outside this region, the ionospheric flows could be greatly enhanced or weakened, while the convection pattern shape changes very little. When By is strong enough, the Bz reorientation causes changes in the flow intensity but not in the shape of the convection pattern. The results show the characteristics of ionospheric convection response during strong By and suggest that the convection reconfiguration is not only determined by the changing Bz but also significantly influenced by the stable and large By.

Solar wind input between substorm onsets during and after the October 18–20, 1995, magnetic cloud

We study an aspect of substorm occurrence during Earth passage of the October 1995 magnetic cloud and the corotating stream overtaking it. The substorms were identified primarily by five latitudinal arrays of ground‐based magnetometers around the globe and the interplanetary observations were from the Wind spacecraft. The aspect we study is the energy and magnetic flux accumulated in the geomagnetic tail between successive substorm onsets, which we calculate by integrating different theoretical and empirical expressions for the solar wind‐magnetosphere coupling rates. These coupling functions, which depend strongly on the interplanetary magnetic field (IMF) north‐south component, Bz, have all been proposed previously as appropriate measures of solar wind input into the magnetosphere and indeed most correlate well with the cross‐polar cap potential. We contrast the effect on magnetospheric substorms of three solar wind states: A 14 hour long period of continuous and strong IMF Bz < 0 and a 16 hour long period of continuous and strong IMF Bz > 0, both states due to the magnetic cloud; and a 22 hour long period of intermittent IMF Bz polarity of large, but weaker amplitude, due to Alfvén waves in the faster stream. We find that (1) the average rate of substorm onset occurrence during the Bz < 0 phase in the cloud is higher than during the intermittent Bz polarity in the wake, by about a factor of 1.4. (2) The long Bz > 0 phase of the cloud elicited no substorm onsets. (3) The total time for which Bz is negative between substorm onsets is less variable than the interonset interval itself, but still varies between ∼ 0.5 hour and ∼ 4 hours. (4) The integral of any individual solar wind‐magnetosphere coupling function between successive substorm onsets is also variable. (5) The interonset integral varies differently for different coupling functions, even for those which measure the same quantity (energy or magnetic flux). (6) For all energy and magnetic flux coupling functions we find a positive correlation between the interonset integral and the value of the coupling function around the time of the preceding substorm onset. The correlation is best for the ε and υBs functions. This result is consistent with those substorm models in which substorm onset occurs at some fixed energy or magnetic flux threshold and where the amount of energy or magnetic flux lost following each substorm onset is proportional to the solar wind input rate at the time of onset.

Study on neutral wind contribution to the electrodynamics in the polar ionosphere using EISCAT CP‐1 data

Energy coupling between the thermosphere, ionosphere and magnetosphere is studied quantitatively through an analysis using the European Incoherent Scatter (EISCAT) Common Program (CP) −1 version H data obtained on May 3, 1988. A negative excursion of the H component in the Tromsø magnetogram occurred during the experiment period, which involved the following two features: (1) the electric potential across the polar cap was expected to be reduced abruptly in association with a sudden change of the interplanetary magnetic field (IMF) Bz polarity from southward to around null and (2) the negative excursion had a relatively long duration of development (about 4 hours), which may drive neutrals to move significantly through ion drag. In order to investigate the energy coupling between the thermosphere and ionosphere, we evaluate quantitatively the electromagnetic energy flux J·E, the Joule heating rate J·E′ (E′ = E + U × B), and the mechanical energy transfer rate U·(J × B), where U is the neutral wind velocity. The CP‐I‐H experiment provides directly or indirectly all quantities above at altitudes of 101 km, 109 km, 119 km, and 132 km. The results are summarized as follows. (1) The amplitude of the neutral wind related electric field U × B varied greatly with altitude, i.e., at altitudes above 119 km it often became larger than 50% of the amplitude of the observed electric field; (2) during the late recovery phase of the negative excursion of the H component of the Tromsø magnetic field, the neutral wind related electric field tended to be canceled with the observed electric field; (3) in the E region the neutral wind mechanical energy transfer rate U·(J × B) is not negligible but is comparable to the Joule heating rate J·E′; and (4) in particular, at higher altitudes (132 km high) the conversion from the neutral wind mechanical energy to the electromagnetic energy occasionally may occur.

Jet Outbursts from Fast Accretion in a Disk with Zebra‐Stripe Magnetic Field

The optical, X-ray, and gamma-ray outbursts, as well as the associated formation of relativistically moving components of parsec-scale jets of some active galactic nuclei (AGNs) are interpreted as dynamical events in a magnetized accretion disk of a massive black hole. Here we discuss the theory and simulation results for a time-dependent, axisymmetric disk accretion model, including the influence of an ordered magnetic field that reverses polarity as a function of radial distance in the disk. The accretion rate of the disk is coupled to the rate of angular momentum and energy outflow in magnetically driven jets originating from the ±z surfaces of the disk. The inward radial accretion speed in the disk (u) is the sum of the familiar viscous term and a magnetic term proportional to r3/2B2z/σ due to the jets, where Bz(r, t) is the field at the midplane threading the disk, and σ(r, t) is the disk's surface mass density. We consider conditions where the magnetic term is dominant, and we derive coupled nonlinear equations for the evolution of Bz and σ. For general initial conditions, Bz and σ vary with r. Furthermore, Bz necessarily reverses polarity in order to conserve flux. As a result of the polarity reversals, the evolution of σ and Bz leads to the formation of inward facing shocks, where the radial derivatives of Bz and σ are very large. The shocks separate different annular regions of the disk threaded by positive and negative Bz. The kinetic luminosity in the jets is predominantly from the innermost part of the disk. Consequently, the passage of a shock through the inner edge of the disk gives a strong, narrow spike in the jet kinetic luminosity. We interpret this spike as an outburst of an AGN and the associated creation of a new parsec-scale jet component. Also in this picture, the outburst corresponds to a reversal of polarity of Bz in the inner part of the disk. As a result of the jet's propagation and radial expansion, this polarity reversal becomes a polarity reversal of Bϕ as z varies across the jet component. Consequently, magnetic field annihilation in the jet component may be important, in particular, for accelerating the leptons to the high Lorentz factors needed to explain the observed synchrotron, synchrotron self-Compton, and inverse Compton radiation.