Interplanetary Field Research Articles

Occurrence of Equatorial Plasma Bubbles during Intense Magnetic Storms

An important issue in low-latitude ionospheric space weather is how magnetic storms affect the generation of equatorial plasma bubbles. In this study, we present the measurements of the ion density and velocity in the evening equatorial ionosphere by the Defense Meteorological Satellite Program (DMSP) satellites during 22 intense magnetic storms. The DMSP measurements show that deep ion density depletions (plasma bubbles) are generated after the interplanetary magnetic field (IMF) turns southward. The time delay between the IMF southward turning and the first DMSP detection of plasma depletions decreases with the minimum value of the IMFBz, the maximum value of the interplanetary electric field (IEF)Ey, and the magnitude of the Dst index. The results of this study provide strong evidence that penetration electric field associated with southward IMF during the main phase of magnetic storms increases the generation of equatorial plasma bubbles in the evening sector.

Storm-time characteristics of intense geomagnetic storms ( [formula omitted]) at low-latitudes and associatedenergetics

Tremendous amount of solar energy is hurled into the space by transient sporadic emissions occurring within the Sun. A fraction of this energy is transferred into the Earth's magnetosphere by the magnetic reconnection process. Interplanetary magnetic field plays a crucial role in the excitation of geomagnetic storms and their subsequent evolution. The present study attempts to determine the influence of post-shock duration of southward B z on the development and intensification of intense ( Dst ≤ − 200 nT ) geomagnetic storms. The study presents 18 big storm events that occurred during the solar cycle 23. In all the cases under study, the interplanetary shocks were driven by the interplanetary coronal mass ejections (ICMEs). The ICME structures may contain southward magnetic fields within the sheath, the magnetic cloud or both in succession, which can lead to the development of intense geomagnetic storms. In addition, dependence of storm strength on the total energy influx into the magnetosphere ( ϵ ) and ring current ( E RC ) energy is also assessed. Geomagnetic storm characteristics are examined at a low-latitude station, Alibag (geographic lat. 18 . 63 ∘ N , long. 72 . 87 ∘ E ; geomagnetic lat. 10 . 02 ∘ N , long. 145 . 97 ∘ ), using high resolution digital data. The minimum duration of southward B z for strengthening the storms is ∼ 1.25 h . All intense storms are found to have minimum values of southward directed B z to be ≤ − 18 nT and interplanetary electric field E y > 12 mV / m . Intensity of geomagnetic storms at low-latitudes follows a fairly linear dependence on the ring current energy.

Polar cap electric field saturation during interplanetary magnetic field Bz north and south conditions

We report the results of an investigation of the saturation of the polar cap electric field during periods of large northward and southward interplanetary magnetic field (IMF). While it has been demonstrated that saturation can occur for both northward and southward IMF, a direct comparison between the two regimes during saturated driving has not been performed. We use solar wind measurements to search for events between 1998 and 2007 when the IMF is stable for 50 min. The selected intervals are binned according to interplanetary electric field (−VSW × B). SuperDARN Doppler radar velocity vectors from high‐latitude antisunward looking beams are averaged to determine the approximate polar cap electric field. Results show that sunward convection under northward IMF is stronger in summer than in winter, but that antisunward convection under southward IMF exhibits the opposite seasonal behavior. One explanation is that, as the earth tilts near solstice, lobe reconnection is less effective in the winter hemisphere.

Electrodynamic coupling processes between the magnetosphere and the equatorial ionosphere during a 5-day HILDCAA event

We present a statistical analysis of equatorial F-peak height vertical drifts during a 5-day HILDCAA event (3–7 July 2003). Prompt penetration-PP drifts have been identified in this study by means of a correlation analysis carried out between the magnitude of disturbed height variations over the equatorial station São Luís-SL (44.6W, 2.33S, dip angle 1.5S) and the magnitudes of the interplanetary electric field at the magnetopause. A comparison between the correlation coefficients for the HILDCAA and the post-HILDCAA days clearly shows that coupling on the post-HILDCAA period is suppressed. The daytime (nighttime) ionospheric responses to magnetopause electric field changes were found to occur within ∼55 min (∼45 min).

Daytime efficiency and characteristic time scale of interplanetary electric fields penetration to equatorial latitude ionosphere

We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the equatorial ionosphere based on a correlation analysis carried out between different levels of decomposition applied to IEF intensity measured at the ACE spacecraft and ionospheric electric field intensity inferred from ground based magnetometers located in the equatorial region in Brazil. We compare the time variations of those two electric field intensities by means of a scale-by-scale decomposed time series through wavelet multi-resolution analysis. Efficiency is here defined as the fraction of the variation of the IEF intensity that has penetrated into the equatorial region, and it increases with increasing fraction. Two cases of prompt penetration electric fields (PPEF) are analyzed: one occurring on March 31, 2001, and other on April 17, 2002. Our results show that the penetration effect with time scale ranging around 1 h is maximized in relation to other scales.

Auroral kilometric radiation diurnal, semidiurnal, and shorter‐term modulations disentangled by Cassini

During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded a month of quasi‐continuous observations of auroral kilometric radiation (AKR). Analyzing the full polarization of incoming radio waves, we found AKR to be 100% circular left‐handed (LH) or right‐handed (RH). As AKR is emitted predominantly on the extraordinary mode, we analyzed separately the northern (RH) emission, from the southern (LH) one with respect to the magnetic equator. AKR power variations reveal (1) a log‐normal distribution at time scales of minutes, (2) bursts of emission at time scales of a few hours, and (3) a clear overall modulation at 24 h, together with a weaker modulation at 12 h (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming as a function of explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anticorrelation (at 24 h) dominates close to Earth, while the short‐term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, namely, for the LH emission, which is observed mostly near the magnetic equator along the path of Cassini. As a 12 h modulation of some geomagnetic indices was also observed, we suggest that the AKR semidiurnal modulation may have a physical cause, for which we favor the variable geometry and efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations.

Storm time response of GPS-derived total electron content (TEC) during low solar active period at Indian low latitude station Varanasi

The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′ N, longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20 November 2007, 9 March 2008 and 11 October 2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence.

Magnetosheath for almost‐aligned solar wind magnetic field and flow vectors: Wind observations across the dawnside magnetosheath at X = −12 Re

While there are many approximations describing the flow of the solar wind past the magnetosphere in the magnetosheath, the case of perfectly aligned (parallel or anti‐parallel) interplanetary magnetic field (IMF) and solar wind flow vectors can be treated exactly in a magnetohydrodynamic (MHD) approach. In this work we examine a case of nearly‐opposed (to within 15°) interplanetary field and flow vectors, which occurred on October 24–25, 2001 during passage of the last interplanetary coronal mass ejection in an ejecta merger. Interplanetary data are from the ACE spacecraft. Simultaneously Wind was crossing the near‐Earth (X ∼ −13 Re) geomagnetic tail and subsequently made an approximately 5‐hour‐long magnetosheath crossing close to the ecliptic plane (Z = −0.7 Re). Geomagnetic activity was returning steadily to quiet, “ground” conditions. We first compare the predictions of the Spreiter and Rizzi theory with the Wind magnetosheath observations and find fair agreement, in particular as regards the proportionality of the magnetic field strength and the product of the plasma density and bulk speed. We then carry out a small‐perturbation analysis of the Spreiter and Rizzi solution to account for the small IMF components perpendicular to the flow vector. The resulting expression is compared to the time series of the observations and satisfactory agreement is obtained. We also present and discuss observations in the dawnside boundary layer of pulsed, high‐speed (v ∼ 600 km/s) flows exceeding the solar wind flow speeds. We examine various generating mechanisms and suggest that the most likely cause is a wave of frequency 3.2 mHz excited at the inner edge of the boundary layer by the Kelvin‐Helmholtz instability.

Comparison of the open‐closed separatrix in a global magnetospheric simulation with observations: The role of the ring current

The development of global magnetospheric models, such as Space Weather Modeling Framework (SWMF), which can accurately reproduce and track space weather processes has high practical utility. We present an interval on 5 June 1998, where the location of the polar cap boundary, or open‐closed field line boundary (OCB), can be determined in the ionosphere using a combination of instruments during a period encompassing a sharp northward to southward interplanetary field turning. We present both point‐ and time‐varying comparisons of the observed and simulated boundaries in the ionosphere and find that when using solely the coupled ideal magnetohydrodynamic magnetosphere‐ionosphere model, the rate of change of the OCB to a southward turning of the interplanetary field is significantly faster than that computed from the observational data. However, when the inner magnetospheric module is incorporated, the modeling framework both qualitatively, and often quantitatively, reproduces many elements of the studied interval prior to an observed substorm onset. This result demonstrates that the physics of the inner magnetosphere is critical in shaping the boundary between open and closed field lines during periods of southward interplanetary magnetic field (IMF) and provides significant insight into the 3‐D time‐dependent behavior of the Earth's magnetosphere in response to a northward‐southward IMF turning. We assert that during periods that do not include the tens of minutes surrounding substorm expansion phase onset, the coupled SWMF model may provide a valuable and reliable tool for estimating both the OCB and magnetic field topology over a wide range of latitudes and local times.

Storm time electric fields in the equatorial ionosphere observed near the dusk meridian

We compare the dynamics of plasma drifts (VP) and ionospheric electric fields (EI), measured near the dusk meridian at the magnetic equator by the Defense Meteorological Satellite Program (DMSP) F13 satellite, with those of the dawn‐to‐dusk component of the interplanetary electric field (IEF) Ey during four large magnetic storms. Presented data indicate that during the main phase of the magnetic storms when the interplanetary magnetic field (IMF) had a southward component, VP had upward and westward components that ranged up to 120 and 210 m s−1. The corresponding eastward and upward components of EI were approximately 5% and 10% of Ey. The persistence of eastward EI for many hours indicates that electric field shielding of the inner magnetosphere is ineffective during prolonged periods of southward IMF. During periods of northward IMF (Ey < 0) ionospheric responses depended on prior history. No ionospheric response is apparent after strong northward IMF turnings preceded by geomagnetic quiet. If they occurred after a long southward interval, EI often had westward and downward components. The polarity of the zonal component of EI remained well correlated with that of Ey for up to 21 h during periods of slowly varying IMF. Phenomena associated with southward IMF periods are consistent with models that allow for prolonged penetration electric field from high latitudes superposed on those of the Sq current system. However, it is not understood why the remarkable correspondence between the polarities of the zonal component of EI and the IEF Ey occurs during northward IMF. In some Ey < 0 periods, the vertical component of EI remained upward. DMSP detections of upward EI (westward VP) during periods of Ey < 0 are not predicted by existing models.

Midday magnetopause shifts earthward of geosynchronous orbit during geomagnetic superstorms with Dst ≤ −300 nT

Geomagnetic storm intensity, as measured by the Dst (SYMH) index, shows no limit as the solar wind dawn‐to‐dusk electric field increases. We show that the magnetopause around noon erodes earthward with increasing storm intensity. The panoramic geosynchronous BZ signatures for the magnetic storm groups with different intensity are differ significantly from each other. For superstorms with SYMH ≤ −300 nT, the magnetopause around noon erodes to inside the geosynchronous orbit, which causes the BZ reversal near local noon. The necessary conditions for superstorms with SYMH ≤ −300 nT to occur include the following: (1) interplanetary magnetic field (IMF) BZ < −27 nT lasts for at least ∼1 h; (2) solar wind dynamic pressure, Pd > ∼ 12 nPa; (3) the projected interplanetary electric field, EK − L > ∼30 mV/m.

Widely different characteristics of oxygen and hydrogen ion escape from Venus

We study the solar wind induced escape of O+ and H+ ions from Venus' atmosphere in the HYB‐Venus hybrid simulation. Most of the previous Venus global plasma modelling studies have concentrated only on the O+ escape. According to the hybrid simulation, planetary O+ and H+ ions behave very differently from each other in the Venusian induced magnetosphere. Both species are asymmetrically distributed in the direction of the interplanetary electric field and in the dawn‐dusk plane. The H+ flow can be understood by E × B drift motion but finite Larmor radius (FLR) effects are essential to the behavior of O+ ions. These differences result in different H+/O+ escape ratios globally and in the plasma wake. Further, the energy ratio of the escaping planetary ions was found to be consistent with the observations made in the near Venus wake by the ASPERA‐4 instrument onboard the Venus Express spacecraft.

Hemispheric asymmetry of the magnetic field wrapping pattern in the Venusian magnetotail

We examine statistically the magnetic field in the Venusian magnetotail which is formed by the draping of interplanetary magnetic field lines. Although the near‐planet and distant magnetotail regions have been sampled by the various missions to Venus and the general magnetic features of the distant magnetotail are well established, the near wake region from about 1.3 to 3 Venusian radii downstream of the planet remained unexplored until the Venus Express mission. Here we report the unanticipated finding of a draped field reversal in one hemisphere of the near Venus tail. When ordered by the interplanetary electric field orientation, the magnetic field lines in the hemisphere with inward motional electric field apparently are wrapped more tightly around Venus than in the other hemisphere, thus forming a field reversal region in the this portion of the near tail. A global hybrid simulation produces what we see and provides a three‐dimensional view of the observed hemispherical asymmetry.

Response of the reconnection electric field and polar cap potential to the IMF and velocity of solar wind

It is well known that the cross polar cap potential is saturated under a strong interplanetary electric field and is often said to be related to the ionospheric currents. To investigate the other factors influencing this phenomenon, a global magnetohydrodynamics simulation not including the feedback from the ionosphere to the magnetosphere was conducted. The simulation results showed that an increase in the southward IMF causes a smaller increase in the cross polar cap potential than that caused by an increase in the solar wind velocity. This difference was caused by the transportation of reconnected magnetic field lines towards the tail.

On the relationship between coronal heating, magnetic flux, and the density of the solar wind

The stark differences between the current solar minimum and the previous one offer a unique opportunity to develop new constraints on mechanisms for heating and acceleration of the solar wind. We have used a combination of numerical simulations and analysis of remote solar and in situ observations to infer that the coronal heating rate, H, scales with the average magnetic field strength within a coronal hole, Bch. This was accomplished in three steps. First, we analyzed Ulysses measurements made during its first and third orbit southern and northern polar passes (i.e., during near‐solar minimum conditions) to deduce a linear relationship between proton number density (np) and radial magnetic field strength (Br) in the high‐speed quiescent solar wind, consistent with the results of McComas et al. (2008) and Ebert et al. (2009). Second, we used Wilcox Solar Observatory measurements of the photospheric magnetic field to show that the magnetic field strength within coronal holes (Bch) is approximately correlated with the strength of the interplanetary field at the location of Ulysses. Third, we used hydrodynamic simulations to show that np in the solar wind scales linearly with H. Taken together, these results imply the chain: H ∝ np ∝ Br ∝ Bch. We also explored ideas that the correlation between np and Br could have resulted from interplanetary processes, or from the superradial expansion of the coronal magnetic field close to the Sun, but find that neither possibility can produce the observed relationship. The derived heating relationship is consistent with (1) empirical heating laws derived for closed‐field line regions and (2) theoretical models aimed at understanding both the heating and acceleration of the solar wind.

Relationship between magnetic activity in the polar cap and atmospheric processes in the winter Antarctica

The paper demonstrates the close relationships between the polar cap magnetic activity, which is characterized by PC index ( Troshichev et al., 1988, 2006) and some atmospheric phenomena typical of the winter Antarctica, such as enhancement of cloudiness, sudden warmings of the ground atmosphere in near-pole area, and formation of anomalous wind regimes above Antarctica. It was shown previously ( Troshichev et al., 2004, 2008; Troshichev and Janzhura, 2004;) that these atmospheric phenomena are controlled by variations of the geoeffective interplanetary electric field impacting the Earth’s magnetosphere. On the other hand, the polar cap magnetic activity is also determined by the interplanetary electric field influence through the field-aligned magnetospheric currents and electric field in the polar cap ionosphere. The results imply that the PC index, available online at http://www.aari.nw.ru from the near-pole station Vostok, can be used to monitor the anomalous atmospheric processes in winter Antarctica.

Analysis of the step response function relating the interplanetary electric field to the dayside magnetospheric reconnection potential

We present a statistical analysis of the response of the magnetic reconnection rate between the interplanetary magnetic field and the magnetosphere to southward turnings of the interplanetary magnetic field. The magnetic reconnection rate is calculated from Super Dual Auroral Radar Network (SuperDARN) measurements. The polar cap boundary is identified as the offset circle (3° toward midnight from the magnetic pole) that best separates high spectral width backscatter, indicative of open magnetic field lines, from low spectral width backscatter. The electric potential on the boundary is determined from the best fit of the line‐of‐sight F region plasma velocity to an eighth‐order spherical harmonic function of ionospheric electrical potential. The reconnection rate is determined from the electric potential on the boundary and the expansion rate of the polar cap. Solar wind data (velocity and magnetic field) are obtained from Wind spacecraft measurements and propagated to the magnetopause. The relationship between interplanetary electric field and the magnetic reconnection rate is analyzed by calculating linear response functions. We determine that the convection term in the reconnection rate measurement exhibits a unimodal response to the interplanetary electric field, whereas the boundary motion term in the reconnection rate measurement exhibits a bimodal response, with a positive mode (increase in polar cap expansion rate) followed by a negative mode. The response of the dayside reconnection rate to a step change in the interplanetary electric field exhibits a transient response of one hour duration followed by a steady state response at a level that is half of the peak response.

Interplanetary field enhancements travel at the solar wind speed

Interplanetary Field Enhancements (IFEs) are cusp‐shaped increases and decreases in the interplanetary field, accompanied by a thin current sheet. They can last many hours and even the strongest have no leading or trailing shocks. These rare structures are of possible significance because their occurrence has been associated with potential sources of charged dust. On December 24, 2006, four well‐separated spacecraft observed an IFE close to the Earth. From these four spacecraft measurements, we find that this IFE was traveling at the solar wind speed. This observation is consistent with previous encounters with fewer multiple satellites but is the first definitive measure of the speed of an IFE. This high speed is consistent with the lack of observation of shocks in association with these structures. At least on this occasion, the IFE was a pressure‐balanced structure in which the thermal plasma pressure gradients balanced the magnetic pressure gradients.

Comparative study of ion cyclotron waves at Mars, Venus and Earth

Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind–Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus.

Interplanetary magnetic field–geomagnetic field coupling and vertical variance index

The solar wind impacting at the geomagnetopause contains transient variations in the embedded interplanetary magnetic field (IMF). These disturbances are mirrored in the horizontal geomagnetic field measured at the Huancayo and Ascension Island stations. This investigation attempts to relate the microtemporal fluctuations in the IMF with the horizontal component of the geomagnetic field by means of a newly developed daily vertical variance index, which permits the quantification of such pulsations. A linear relationship is established between the disturbances observed in the interplanetary and terrestrial fields on a daily basis.