C. Cid

PLASMA AND MAGNETIC FIELD INSIDE MAGNETIC CLOUDS: A GLOBAL STUDY

Data observed during spacecraft encounters with magnetic clouds have been extensively analyzed in the literature. Moreover, several models have been proposed for the magnetic topology of these events, and fitted to the observations. Although these interplanetary events present well-defined plasma features, none of those models have included a simultaneous analysis of magnetic field and plasma data. Using as a starting point a non-force-free model that we have developed previously, we present a global study of MCs that include both the magnetic field topology and the plasma pressure. In this paper we obtain the governing equations for both magnitudes inside a MC. The expressions deduced are fitted simultaneously to the measurements of plasma pressure and magnetic field vector. We perform an analysis of magnetic field and plasma WIND observations within several MCs from 1995 to 1998. The analysis is confined to four of these events that have high-quality data. Only in one fitting procedure we obtain the orientation of the magnetic cloud relative to the ecliptic plane and the current density of the plasma inside the cloud. We find that the equations proposed reproduce the experimental data quite well.

Hyperbolic decay of the Dst Index during the recovery phase of intense geomagnetic storms

What one commonly considers for reproducing the recovery phase of magnetosphere, as seen by the Dst index, is exponential function. However, the magnetosphere recovers faster in the first hours than in the late recovery phase. The early steepness followed by the late smoothness in the magnetospheric response is a feature that leads to the proposal of a hyperbolic decay function to reproduce the recovery phase, instead of the exponential function. A superposed epoch analysis of recovery phases of intense storms from 1963-2006 was performed, categorizing the storms by their intensity into five subsets. The hyperbolic decay function reproduces experimental data better than what the exponential function does for any subset of storms, which indicates a non-linear coupling between dDst/dt and Dst. Moreover, this kind of mathematical function, where the degree of reduction of the Dst index depends on time, allows for explaining different lifetimes of the physical mechanisms involved in the recovery phase and provides new insights for the modeling of the Dst index.

Three frontside full halo coronal mass ejections with a nontypical geomagnetic response

After analyzing the source regions of these halo CMEs, it was found that the halo associated with the strongest geomagnetic disturbance was the one that initiated farther away from disk center (source region at W66); while the other two CMEs originated closer to the central meridian but had weaker geomagnetic responses. Therefore, these three events do not fit into the general statistical trends that relate the location of the solar source and the corresponding geoeffectivity. We investigate possible causes of such a behavior. Nonradial direction of eruption, passage of the Earth through a leg of an interplanetary flux rope, and strong compression at the eastern flank of a propagating interplanetary CME during its interaction with the ambient solar wind are found to be important factors that have a direct influence on the resulting north-south interplanetary magnetic field (IMF) component and thus on the CME geoeffectiveness. We also find indications that interaction of two CMEs could help in producing a long-lasting southward IMF component. Finally, we are able to explain successfully the geomagnetic response using plasma and magnetic field in situ measurements at the L1 point. We discuss the implications of our results for operational space weather forecasting and stress the difficulties of making accurate predictions with the current knowledge and tools at hand.

Modeling the recovery phase of extreme geomagnetic storms

The recovery phase of the largest storms ever recorded has been studied. These events provide an extraordinary opportunity for two goals: (1) to validate the hyperbolic model by Aguado et al. [2010] for the recovery phase after disturbances as severe as the Carrington event, or that related to the Hydro-Quebec blackout in March 1989, and (2) to check whether the linear relationship between the recovery time and the intensity of the storm still complies. Our results reveal the high accuracy of the hyperbolic decay function to reproduce the recovery phase of the magnetosphere after an extreme storm. Moreover, the characteristic time that takes the magnetosphere to recover depends in an exponential way on the intensity of the storm, as indicated by the relationship between the two parameters involved in the hyperbolic decay. This exponential function can be approached by a linear function when the severity of the storm diminishes.

Comment on “Interplanetary conditions leading to superintense geomagnetic storms (Dst ≤ −250 nT) during solar cycle 23” by E. Echer et al.

[1] Echer et al. [2008] studied the interplanetary causes of superintense (Dst 250 nT) geomagnetic storms that occurred during solar cycle 23. From a sample of 11 events (listed in Table 1 of Echer et al. [2008], hereinafter referred to as Echer Table), they found that 1/3 of the superstorms are caused by MC (magnetic cloud) fields, 1/3 by a combination of SH (sheath) + MC fields and 1/3 by SH fields. From these results, joint to a study by Tsurutani et al. [1992] for the five greatest storms in the period 1971–1986, they concluded that ‘‘only MC and sheath fields seems to be important causes for the development of superstorms’’. Thus, corotating interaction regions (CIRs) or heliospheric current sheet (HCS) fields are not causes of superstorms. Moreover, Echer et al. [2008] concluded that ‘‘there is a higher probability of single structures causing the events’’. [2] Nevertheless, several papers reported complex interplanetary structures as drivers of severe geomagnetic activity. Wang et al. [2003a] found out that two of three Multi-MCs (multiple magnetic cloud, which is formed by the overtaking of successive CMEs), are associated with the great geomagnetic storms (Dst 200 nT). Analyzing longlived geomagnetic storms Xie et al. [2006] concluded that the intensity of large geomagnetic storms is well-related to the degree of interaction (the number of interplanetary coronal mass ejections –ICMEs– interacting with a high speed stream –HSS– event or with themselves). [3] Huttunen et al. [2002] studied the event of April 7, 2000 (event 1 of Echer Table) and they concluded that the fluctuating but strongly southward field accompanied by the high pressure allowed for the exceptionally strong driving magnetospheric activity. A high speed stream from a coronal hole interacting with the ‘magnetic cloud like’ was reported by Xie et al. [2006] for this event, resulting the enhanced pressure inside the ICME which causes great geomagnetic activity. [4] The paper of Wang et al. [2003a] shows a detailed analysis of the event of March 31, 2001 (event 3 of Echer Table). Wang et al. [2003a, Figure 2] shows clearly two MCs with an interacting region between them, and another small ejecta as the interplanetary cause of this geomagnetic storm. For this event, Xie et al. [2006] described the interplanetary driver causing southward as ‘magnetic cloud like’ and added as a comment that four CMEs were involved in the interaction. Zhang et al. [2007a, 2007b] also reported multiple structures of type SH + MC cloud involved in this geomagnetic storm, as well as in the event of April, 12, 2001 (event 4 of Echer Table). Wang et al. [2003a] also analyzed this last event and found that several interacting MCs are indeed the interplanetary driver of the geomagnetic activity. Wang et al. [2003a, Figure 3] show Ace observations from 11 April to 14 April 2001, which are carefully described in Section 4 of the paper. [5] Xie et al. [2006] identified the interplanetary driver of the geomagnetic storm of November 6, 2001 (event 5 of Echer Table) with a SH + compressed ICME + HSS. They also stated that 3 halo CMEs were participating in the event. Zhang et al. [2007a, 2007b] identified the interplanetary sources of this event as MC + PMC-SH (a shock propagating through a preceding magnetic cloud) + ICME, although they commented that there were optional choices of solar sources and an EIT data gap. Figure 1 shows ACE spacecraft data from November 5 to November 7, 2001. Two solid lines have been drawn in order to show the main phase of this geomagnetic storm. There is no doubt that the interplanetary event associated is a complex structure and, although a solar wind data gap appears, two interplanetary shocks (S1 and S2 in Figure 1) and some regions with smooth and elevated magnetic field can be identified. A sharp decrease in proton temperature and density is also evident at November 5 19:35 UT, indicating the boundary of an ICME, which magnetic signatures guided Wang et al. [2003b] to consider the first shadowed region as a MC. Although solar wind data are missing, an ejecta can be also guessed in the second shadowed area, driving the shock S2 which overtakes the first magnetic cloud. Wang et al. [2003b] pointed out that the compression between the overtaking shock and the preceding MC increased the geoeffectiveness of this event. [6] The Dst profile (Figure 1 (bottom)) shows a complex development, where at least two intense dips can be noticed, departing from a classical ‘‘main-recovery’’ phase development. The number of peaks in Dst is not necessarily directly related to the number of interplanetary transients that are involved in generating the storm [Richardson and Zhang, 2008]. However, in this case, after the initial phase of the storm, related to the shock (S1) and sheath, the main phase GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L21107, doi:10.1029/2008GL034731, 2008 Click Here for Full Article

Supergranular-scale magnetic flux emergence beneath an unstable filament

Here we report evidence of a large solar filament eruption on 2013, September 29. This smooth eruption, which passed without any previous flare, formed after a two-ribbon flare and a coronal mass ejection towards Earth. The coronal mass ejection generated a moderate geomagnetic storm on 2013, October 2 with very serious localized effects. The whole event passed unnoticed to flare-warning systems. We have conducted multi-wavelength analyses of the Solar Dynamics Observatory through Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) data. The AIA data on 304, 193, 211, and 94 \AA sample the transition region and the corona, respectively, while HMI provides photospheric magnetograms, continuum, and linear polarization data, in addition to the fully inverted data provided by HMI. [...] We have observed a supergranular-sized emergence close to a large filament in the boundary of the active region NOAA11850. Filament dynamics and magnetogram results suggest that the magnetic flux emergence takes place in the photospheric level below the filament. Reconnection occurs underneath the filament between the dipped lines that support the filament and the supergranular emergence. The very smooth ascent is probably caused by this emergence and torus instability may play a fundamental role, which is helped by the emergence.

Evidence of Magnetic Flux Ropes in the Solar Wind From Sigmoidal and non-Sigmoidal Active Regions

We have examined WIND magnetic field and plasma data during the first half of 1998 in order to find encounters of this spacecraft with magnetic clouds. From the events obtained through this search, we have selected four of them taking into account their solar origin. The four magnetic clouds are related to halo or partial halo CMEs, but the morphology of the active region before the eruption is sigmoidal for three of them and non-sigmoidal for the other one. We have analyzed these events in the solar wind by fitting the experimental data to a non-force-free flux-rope model. We conclude that both kinds of active regions develop in the solar wind an ejection with a flux-rope topology.

Photospheric magnetic field of an eroded-by-solar-wind coronal mass ejection

We have investigated the case of a coronal mass ejection that was eroded by the fast wind of a coronal hole in the interplanetary medium. When a solar ejection takes place close to a coronal hole, the flux rope magnetic topology of the coronal mass ejection (CME) may become misshapen at 1 AU as a result of the interaction. Detailed analysis of this event reveals erosion of the interplanetary coronal mass ejection (ICME) magnetic field. In this communication, we study the photospheric magnetic roots of the coronal hole and the coronal mass ejection area with HMI/SDO magnetograms to define their magnetic characteristics.

Flux emergence event underneath a filament

Flux emergence phenomena are relevant at different temporal and spatial scales. We have studied a flux emergence region underneath a filament. This filament elevated itself smoothly, and the associated CME reached Earth. In this study we investigate the size and amount of flux in the emergence event. The flux emergence site appeared just beneath a filament. The emergence acquired a size of 24 Mm in half a day. The unsigned magnetic flux density from LOS-magnetograms is around 1 kG at its maximum. The transverse field as well as the filament eruption were also analysed.

The Geoeffectiveness of Magnetic Clouds as a Function of Their Orientation

Trying to get light into the paradigm of forecasting geomagnetic activity, we have looked for a relationship between geoeffectiveness and the orientation and helicity of magnetic clouds. During the years 1995–2000, we have selected all the geomagnetic storms with Dst index less than −70 nT. Then, we have inspected WIND data looking for a possible magnetic cloud related to every storm event. When a magnetic cloud is encountered, we have fitted to experimental data a model that we have developed for the magnetic cloud topology in order to obtain the attitude of the magnetic cloud and its helicity. On the basis of the results obtained, a close relationship is observed between the orientation of the magnetic cloud and its helicity, and the geomagnetic activity.