Southward Component Of Interplanetary Magnetic Field Research Articles

Solar rotation and solar wind–magnetosphere coupling

This paper deals with three characteristics of the interplanetary magnetic field (IMF), important for solar wind–magnetosphere coupling and related to solar rotation: the IMF azimuthal component, the IMF total magnitude, and the handedness or the sense of rotation of magnetic clouds. The IMF configuration is described by Parker's Archimedian spiral model (Astrophys. J. 128 (1958) 664) under the assumptions of a purely radial solar wind with a constant velocity emanating from a uniformly rotating Sun. In situ measurements confirmed this general picture, but a systematic deviation from the predicted IMF winding angle was found, supposedly exhibiting a 11-year periodicity. We account for the non-uniform solar rotation and compare the observed IMF azimuthal component to the one calculated from Parker's formula with the measured equatorial solar rotation rate. We find that the differences between the calculated and measured IMF azimuthal component and the winding angle have a clear 22-year dependence on the solar polarity cycle, matching the 22-year periodicity in solar rotation rate rather than on the 11-year sunspot cycle. Our results are an observational confirmation of the validity of the model of Fisk (J. Geophys. Res. 101 (1996) 15547) for heliospheric magnetic field with footpoint motions due to solar differential rotation. Solar differential rotation is also an important element of the solar dynamo which is responsible for the generation of the solar magnetic field. We compare the different periodicities in the variations in the latitudinal rotation gradient of the two solar hemispheres and show that the IMF which is an extension of the solar coronal field, is related to the differential rotation in the more active solar hemisphere. Another feature related to solar differential rotation that is persistently different in the two solar hemispheres is the prevailing magnetic helicity, which is carried to the Earth by magnetic clouds preserving the helicity of the source region of their origin. The reaction of the magnetosphere to magnetic clouds is determined mainly by the presence or absence of a prolonged period of southward IMF component. We show that it also depends on the helicity of the clouds, and compare the effects of right- and left-handed magnetic clouds on geomagnetic activity.

Azimuthally asymmetric ring current as a function of <i>D<sub>st</sub></i> and solar wind conditions

Abstract. Based on magnetic data, spatial distribution of the westward ring current flowing at |z|<3 RE has been found under five levels of Dst, five levels of the interplanetary magnetic field (IMF) z component, and five levels of the solar wind dynamic pressure Psw. The maximum of the current is located near midnight at distances 5 to 7 RE. The magnitude of the nightside and dayside parts of the westward current at distances from 4 to 9 RE can be approximated as Inight=1.75-0.041 Dst, Inoon=0.22-0.013 Dst, where the current is in MA. The relation of the nightside current to the solar wind parameters can be expressed as Inight=1.45-0.20 Bs IMF + 0.32 Psw, where BsIMF is the IMF southward component. The dayside ring current poorly correlates with the solar wind parameters.

IMF control of cusp proton emission intensity and dayside convection: implications for component and anti-parallel reconnection

Abstract. We study a brightening of the Lyman-a emission in the cusp which occurred in response to a short-lived south-ward turning of the interplanetary magnetic field (IMF) during a period of strongly enhanced solar wind plasma concentration. The cusp proton emission is detected using the SI-12 channel of the FUV imager on the IMAGE spacecraft. Analysis of the IMF observations recorded by the ACE and Wind spacecraft reveals that the assumption of a constant propagation lag from the upstream spacecraft to the Earth is not adequate for these high time-resolution studies. The variations of the southward IMF component observed by ACE and Wind allow for the calculation of the ACE-to-Earth lag as a function of time. Application of the derived propagation delays reveals that the intensity of the cusp emission varied systematically with the IMF clock angle, the relationship being particularly striking when the intensity is normalised to allow for the variation in the upstream solar wind proton concentration. The latitude of the cusp migrated equatorward while the lagged IMF pointed southward, confirming the lag calculation and indicating ongoing magnetopause reconnection. Dayside convection, as monitored by the SuperDARN network of radars, responded rapidly to the IMF changes but lagged behind the cusp proton emission response: this is shown to be as predicted by the model of flow excitation by Cowley and Lockwood (1992). We use the numerical cusp ion precipitation model of Lockwood and Davis (1996), along with modelled Lyman-a emission efficiency and the SI-12 instrument response, to investigate the effect of the sheath field clock angle on the acceleration of ions on crossing the dayside magnetopause. This modelling reveals that the emission commences on each reconnected field line 2–2.5 min after it is opened and peaks 3–5 min after it is opened. We discuss how comparison of the Lyman-a intensities with oxygen emissions observed simultaneously by the SI-13 channel of the FUV instrument offers an opportunity to test whether or not the clock angle dependence is consistent with the "component" or the "anti-parallel" reconnection hypothesis.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; solar wind-magnetosphere interactions) – Space plasma physics (magnetic reconnection)

Ultraviolet insolation drives seasonal and diurnal space weather variations

We present several findings that improve the understanding of the seasonal and diurnal variation in auroral and magnetospheric activity. The total ionospheric conductivity in the nightside auroral oval from UV insolation (ΣP) is calculated, and its seasonal and diurnal variation is shown to correlate very highly with that of the Am and AL indices of geomagnetic activity (r = 0.89 and r = 0.75, respectively). Such excellent correlations with Am have been previously obtained by other researchers using instead the acute angle between the Earth's dipole axis and the Earth‐Sun line, ψ. However, the ionospheric conductivity formulation provides a more physical model to explain the equinoctial (McIntosh) effect. Namely, the level of geomagnetic activity is well‐ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither. We improve calculations of the expected pattern of seasonal and diurnal variations in the solar wind input. The elliptical nature of the Earth's orbit results in observed interplanetary magnetic field (IMF) strengths about 7% larger in January than June. When the Sun's spin axis tilt to the ecliptic plane is considered, the predicted IMF southward component (Bs) maximizes in February, as is observed. We also calculate the seasonal and diurnal variation of a more general solar wind–magnetosphere coupling function, EKL. EKL proves to have little (0.5%) diurnal variation and has a seasonal variation of about 14%. For the first time, the seasonal and diurnal variation in the ΦPC, the polar cap flux (from Polar UVI observations, cross‐calibrated to a DMSP‐based standard) and in magnetotail stretching (the b2i index) are presented. Magnetotail stretching proves to correlate better (r = −0.57) with EKL than with ΣP. ΦPC correlates better with ΣP, but the correlation (r = −0.49) is not nearly as strong as that for the indices of geomagnetic activity, Am and AL. Our survey of the seasonal and diurnal variation of the magnetosphere thus shows that some aspects (geomagnetic indices) correlate best with UV insolation, while others (magnetotail stretching) correlate best with solar wind input.

Global electrodynamics observed during the initial and main phases of the July 1991 magnetic storm

Data acquired by seven fortuitously positioned satellites in the interplanetary medium, the magnetosphere, and the topside ionosphere, as well as from numerous ground magnetometers have allowed us to follow the evolution of global currents and electric fields during the geomagnetic storm on July 8–9, 1991. An interplanetary disturbance collided with the magnetosphere at ∼1636 UT on July 8th, compressing the magnetopause inside of geostationary orbit for about 3 hours, as observed by magnetometers on two GOES satellites. The resulting storm developed in four segments with a 7 hour lull separating the initial and main phases. The initial phase was characterized by (1) an extensive, DP 2 current system that produced an AE perturbation of ∼3500 nT, with reporting stations on the dayside, (2) a twenty‐fold increase in auroral electron fluxes, and (3) the immediate appearance of electric fields in the inner magnetosphere. The main phase intensification and earthward movement of the ring current are associated with southward turnings of the interplanetary magnetic field (IMF) and a polar cap potential increase above 200 kV. A significant fraction of the ring current growth was observed prior to the first of several substorms that occurred during the main and recovery phases. The beginning of recovery was characterized by a diminution of the southward IMF component, the cross polar cap potential and magnetospheric electric fields. It was also marked by a substorm onset. Considering the timing of Dst modulations relative to substorm occurrence, we conclude that during this magnetic storm, the electric fields responsible for ring current transport/energization were mostly associated with the DP 2 current system. Finally, in the evening‐sector penetration electric fields of the initial and main phases led to the formation of upward moving equatorial plasma bubbles that were detected by two DMSP satellites in the topside ionosphere.

Global simulation of magnetospheric space weather effects of the Bastille Day storm

We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection due to the strong southward interplanetary magnetic field component B z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted fluctuations are slightly too high.

Distortion of the interplanetary magnetic field by three‐dimensional propagation of coronal mass ejections in a structured solar wind

A three‐dimensional (3‐D) numerical magnetohydrodynamic model is used to investigate the temporal and spatial evolution of large‐scale solar wind (SW) disturbances. A tilted dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating SW flow with a monopolar interplanetary magnetic field (IMF) is established by dynamic relaxation between 0.14 and 1.04 AU. Time‐dependent variation of the pressure and velocity at the inner boundary is applied to simulate injection of a coronal mass ejection (CME) near the streamer belt. In this model, magnetic disturbances arise solely from distortion of the spiral IMF by the bulk motion of CME material; embedded magnetic structures, such as magnetic clouds or flux ropes, are not considered. Numerical results show that the motion and appearance of a CME are affected by its interaction with the velocity and density structure of the background SW. This 3‐D dynamic interaction also affects the orientation and magnitude of the IMF. Substantial excursions of the southward IMF component Bz are generated by three distinct mechanisms: shock compression of the IMF where the shock front is inclined to field lines, draping around the CME by mass flow convection, and distortion of the IMF by rarefaction waves trailing the CME. The geoeffectiveness of the disturbance depends on the initial position of the CME with respect to the streamer belt as well as on the position of Earth with respect to the pulse centerline.

Polar cap area and boundary motion during substorms

The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) orientations during three substorms in January 1997. We measured changes in the polar cap area and motion of the poleward and equatorward boundary of the auroral oval. It was found that the polar cap boundary is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight, and the fading of luminosity below the instrument sensitivity threshold. Generally, these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. A new feature is that the polar cap region clears of precipitation during the substorm growth phase, which expands the size of the polar cap but is not necessarily related to an expansion of the open flux region. Another finding is that the increase in polar cap area prior to onset can be independent of the strength of the southward IMF component. For one case the polar cap area increased while the southward component of the IMF was 0 ± 0.5 nT. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

POLAR magnetic observations of the low‐altitude magnetosphere during the January 1997 coronal mass ejection/magnetic cloud event

During the January 1997 coronal mass ejection/magnetic cloud event, the POLAR spacecraft experienced three successive perigee passes with dramatically different interplanetary magnetic field (IMF) and solar wind conditions on January 9, 10 and 11. The magnetic field observations during these three successive perigee passes are used to examine the response of the low‐altitude polar and auroral magnetosphere to different IMF and solar wind conditions. We find that field‐aligned currents are controlled mainly by the southward IMF component. They are greatly enhanced during strongly southward IMF conditions on January 10, but the solar wind dynamic pressure has little effect on their strength during the passage of the high‐density solar wind filament on January 11. Thus, drag on the magnetosphere is principally due to reconnection at the magnetopause, with little due to viscous effects. Effects of the ring current and the magnetopause current can be identified in the magnetic field data in the low‐altitude magnetosphere. During the magnetic storm of January 10, the ring current exhibits strong dawn‐dusk asymmetry. It is much stronger in the dusk sector than in the dawn sector.

Geomagnetic activity and wind velocity during the Maunder minimum

Abstract. Following a given classification of geomagnetic activity, we obtained aa index values for the Maunder minimum (1645–1715). It is found that the recurrent and fluctuating activities were not appreciable and that the shock activity levels were very low. The aa index level was due almost entirely to the quiet days. Calculated average solar-wind velocities were 194.3 km s–1 from 1657 to 1700 and 218.7 km s–1 from 1700 onwards. Also, the coronal magnetic field magnitude and southward interplanetary magnetic field component Bz were lower. It is concluded that the nearly absent levels of geomagnetic activity during this period were due to lower coronal and Bz magnetic field magnitudes as well as to the continuous impinging on the Earth of a slow wind.

Geomagnetic activity and wind velocity during the Maunder minimum

Following a given classification of geomagnetic activity, we obtained aa index values for the Maunder minimum (1645–1715). It is found that the recurrent and fluctuating activities were not appreciable and that the shock activity levels were very low. The aa index level was due almost entirely to the quiet days. Calculated average solar-wind velocities were 194.3 km s–1 from 1657 to 1700 and 218.7 km s–1 from 1700 onwards. Also, the coronal magnetic field magnitude and southward interplanetary magnetic field component Bz were lower. It is concluded that the nearly absent levels of geomagnetic activity during this period were due to lower coronal and Bz magnetic field magnitudes as well as to the continuous impinging on the Earth of a slow wind.

Tearing instability, Kelvin‐Helmholtz instability, and magnetic reconnection

Two closely related macroscopic instabilities at the magnetospheric boundary layer are the tearing instability and the Kelvin‐Helmholtz (KH) instability. It has been suggested that a coupling of these modes may lead to larger reconnection rates and thus may explain magnetic flux transfer events at the dayside magnetopause. In the framework of two‐dimensional compressible MHD simulations we study the linear and nonlinear evolution of these instabilities. When the velocity of shear flow is below the Alfvén speed, it is verified that the tearing mode is stabilized by the flow shear and the reconnection rate is reduced. For super‐Alfvénic shear velocities the tearing mode ceases its growth, and the KH instability develops consistently with analytical results. A reasonably low resistivity has no significant influence on the growth of the KH mode. Only for nonlinear amplitudes of the KH mode is reconnection driven by the KH vortices. Thus our results do not confirm a linear interaction between the KH and the tearing mode in terms of vortex‐induced reconnection in two dimensions. Particularly, a reasonably small resistivity does not lower the threshold velocity for the KH instability to a level at which it could be excited for typical magnetopause conditions. Therefore the evolution of the KH mode (and vortex‐induced reconnection) with k vectors aligned with a southward interplanetary magnetic field component appears rather unrealistic for the subsolar magnetopause where the magnetosheath flow is slow.

Magnetic flux redistribution in the storm time magnetosphere

The commonly used formula H = DCF + DR, where DCF and DR are the effects of the magnetopause and ring current, respectively, neglects contribution of the cross‐tail current to the Dst variation. The formula allows us to explain satisfactorily the observed relation of the Dst variation to the ring current intensity but faces difficulties in explaining other experimental facts. First, the equatorward shift of the auroral oval cannot be caused by the sole enhancement of the ring current. Second, the observed relation of the Dst growth rate to the southward IMF component [Burton et al., 1975] does not have any quantitative explanation up to now. We suggest using a different formula, H = (2μ0psw)1/2 + DR‐Fout/2S. The formula is obtained from the conditions of magnetic flux conservation and pressure balance. The flux Fout is directed mainly to the nightside of the magnetosphere. Hence the term Fout/2S describes the effect of the crosstail current and a part of the magnetopause currents. During quiet periods, each term in the right‐hand side of our formula is of the order of tens of nanoteslas. During storm time, each term can rise to hundreds of nanoteslas. The flux Fout grows after the interplanetary magnetic field (IMF) becomes southward owing to the flux transport from the dayside to the magnetotail. The growth rate is described by the formula dFout/dt = U − Fout/τF + ηF, where U is the electric potential difference between the dawnside and duskside of the magnetosphere and τF and ηF are constant. The voltage U depends linearly on the IMF southward component. Combining the latter formula with the expression for H yields a relationship between the Dst growth rate and the IMF southward component close to the observed one. Since the auroral oval is mapped predominantly to the plasma sheet of the magnetotail, the growth of Fout during a storm allows us to explain the equatorward shift of the auroral oval. Another prediction from our theory is the erosion of the stable trapping region in which the equatorial cross section S is related to the flux Fout by the equation S1/2[S(2μ0psw)1/2 + Fout] = 3π3/2(ME + MRC), where ME and MRC are the magnetic moments of the Earth and ring current, respectively. Growth of Fout leads to the decrease of S and to the earthward movement of the dayside magnetopause. During storms this effect can be stronger than that of the region 1 Birkeland current, also moving the magnetopause earthward.

Prediction of Geomagnetic Disturbance Profile from Interplanetary Shock Wave Energy Transfer Index, Fs, in Interplanetary Space.

Based on 297 interplanetary shock waves and corresponding geomagnetic disturbances, the relation between their structures has been studied. For the case in which the interplanetary magnetic field lies basically in the ecliptic plane, we found that (1) the temporal profile of geomagnetic disturbance is mainly determined by the plasma state of the shock waves, especially, the closed relation exists between the plasma temperature profile and the recovery phase of geomagnetic disturbances; (2) shock wave energy transfer index, Fs, as defined in the paper, can give a good prediction of the temporal profiles of geomagnetic disturbances. Thus, it may be deduced that there is a basic energy transfer mechanism associated with plasma processes, which would control geomagnetic disturbance profiles, when a southward-component of interplanetary magnetic field is not significant.

Rayed Southern Lights Observed at Belgrano Station: A Study.

The main purpose of this paper is to study the temporal evolution of the auroral electrojet indices one hour before and two hours after the instant of the sudden appearance of rayed southern lights (RAD5). We considered the observations of the RADs as the snapshot taken with a single all-sky camera placed at Belgrano Station (78.8°S, 38.8°W). Thirty three events were studied to exemplify the tendency of both electrojet currents to see if they manifest exponential shapes The above mentioned cases were split according to the signature of the AE index around the moment of the observed RADs. We corroborate the relationship between the southward interplanetary magnetic field component and the occurrence of auroral substorms. We confirmed the tendency of the total and the westward electrojet current intensities to increase and decrease according to an exponential time function.

Interaction of fast steady flow with slow transient flow: A new cause of shock pair and interplanetary Bz event

The occurrence of the nonspiral magnetic field, high helium density, “cold magnetic enhancement” and counterstreaming suprathermal electron flux in the slow flow around a forward shock indicates that the slow flow is the interplanetary counterpart of the coronal mass ejection (ICME). The characteristics of the field and plasma in the fast flow around the reverse shock are typical for a fast steady flow. Thus the shock pair here appears to be caused by interaction of a fast steady flow with a slow ICME. The fact that the slow ICME possesses a planar magnetic structure and a large −Bz component suggests that the slow ICME may be disconnected from the Sun. It is shown that compression alone appears to be adequate to explain the large southward interplanetary magnetic field component within the shocked slow plasma because of the large southward field component present in the ICME ahead of the forward shock. In addition, a new method to infer the shock angle and Mach number from the observed upstream plasma β and the jump ratios of proton density and total magnetic flux density across a shock is suggested.

An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the Earth's magnetosphere during southward interplanetary magnetic field

The interaction between the solar wind and the earth's magnetosphere has been studied by using a time‐dependent three‐dimensional magnetohydrodynamic model in which the interplanetary magnetic field (IMF) pointed in several directions between dawnward and southward. When the IMF is dawnward, the dayside cusp and the tail lobes shift toward the morningside in the northern magnetosphere. The plasma sheet rotates toward the north on the dawnside of the tail and toward the south on the duskside. For an increasing southward IMF component, the plasma sheet becomes thinner and subsequently wavy because of patchy or localized tail reconnection. At the same time the tail field‐aligned currents have a filamentary layered structure. When projected onto the northern polar cap, the filamentary field‐aligned currents are located in the same area as the region 1 currents with a pattern similar to that associated with auroral surges. Magnetic reconnection also occurs on the dayside magnetopause for southward IMF. The steady dayside reconnection mainly occurs in the antiparallel merging region nearest the subsolar point and drives strong convection near the magnetopause as reconnected field lines flow tailward. This causes an enhancement of the polar cap and region 1 field‐aligned currents. The polar cusp field‐aligned currents evolve from the polar cap (NBz) field‐aligned currents as the IMF is rotated from northward to southward.

Self‐consistent theory of three‐dimensional convection in the geomagnetic tail

The self‐consistent theory of time‐dependent convection in the earth's magnetotail of Schindler and Birn (1982) is extended to three dimensions to include more realistic tail geometry and three‐dimensional flow. We confirm that a steady state solution implies unrealistic tail geometry or large particle or energy losses that are unrealistic during quiet times and conclude therefore that as in the 2‐dimensional case the magnetotail becomes time‐dependent for typical convection electric fields. Explicit solutions are derived, even analytically, for the three‐dimensional flow and the electric and magnetic field in a realistic tail geometry, and quantitative examples are presented. Consequences of time‐dependent convection are demonstrated considering two idealized cases of magnetosphere response to solar wind changes: (1) uniform compression as the likely consequence of increasing (static, dynamic or magnetic) solar wind pressure; and (2) compression only in the z direction perpendicular to the plasma sheet as the probable consequence of a dawn to dusk external electric field (Ey > 0), corresponding to a southward interplanetary magnetic field component (Bz < 0). It is found that in case 1 the stability properties do not change very much in time, while case 2 leads to an evolution toward instability, consistent with the observed correlation of Bz < 0 or Ey > 0 with geomagnetic activity. Several other features, already present in the 2‐dimensional theory, are confirmed.

Case studies of the storm time variation of the polar cusp

The latitudinal variations of the polar cusp region were examined during three intense geomagnetic storms. The variations were compared with the intensity of storm time ring current inferred from the Dst index, with the magnitude of the north‐south component Bz of the interplanetary magnetic field and with substorm activity. The common feature is that the rapid equatorward shift occurred during the increase of the ring current growth and during the southward turning of the interplanetary magnetic field orientation. The equatorwardmost latitude of the cusp was reached before the peak of the ring current intensity, by a few to several hours, coinciding with the occurrence of the largest magnitude of the southward interplanetary magnetic field component. However, details of the polar cusp latitudinal movement differ from storm to storm. During the three storms studied, the poleward recovery commenced at the peak magnitude of the negative IMF Bz component, but the recovery proceeded without a clear relation to variations of the interplanetary Bz component, to the ring current intensity, or to the substorm activity. The lowest cusp latitude observed was at ∼61.7°, and the magnitude of this shift seems to be related to the magnitudes of −Bz. It is further observed that the approximate rates of the cusp macroscopic equatorward and poleward movements are about 3° and 1.5° per hour, respectively.

Impulse response of geomagnetic indices to interplanetary magnetic field.

The impulse response of the geomagnetic indices (Dst, AL, AU and AE) to the interplanetary magnetic field southward component (IMF-Bz) is calculated on the assumption that the magnetosphere acts as a linear system. Hourly data for 300 days are used in this analysis. The main results obtained are as follows: (1) Using interplanetary data, a fairly large portion of geomagnetic disturbances may be predicted with the assumption of a linear system. (2) The response of the Dst index and the AL index develops at two steps or has an oscillating property with a period of several hours. (3) The response of the AU index is rather different from that of the other indices. These results may suggest that the disturbances represented by the Dst and AL indices have a common origin associated with the interplanetary magnetic or electric field, whereas those indicated by the AU index have an origin somewhat different from it.