B. Mendoza

Association of coronal mass ejections with the heliomagnetic current sheet

The distribution of distances of coronal mass ejections (CMEs) with respect to the heliomagnetic current sheet is studied. The analysis is carried out for different times during the last solar cycle. We studied first the site of occurrence of all the CMEs for 1980 and 1984-1986. We further considered those CMEs which are associated with interplanetary shocks for the period 1979-1982. Finally, we considered two-ribbon long duration soft X ray events not related with interplanetary shocks and presumably associated with CMEs. We found that during solar minimum the CMEs in general seem better associated with the heliomagnetic current sheet than at solar maximum and that the two-ribbon not shock-related events are poorly associated with it.

Temperature variability in central Mexico and its possible association to solar activity

Abstract Minimum extreme temperature variability from five meteorological stations in the central part of Mexico covering a period from ∼1920 to ∼1990 is examined. We found a correlation coefficient (r=0.65) between these temperature records and geomagnetic activity. Furthermore, by performing spectral analysis peaks were obtained with similar periodicities to those found in the sunspot number, the magnetic solar cycle, cosmic ray fluxes and geomagnetic activity; all of these phenomena are modulated by solar activity. Signals with periodicities comparable to those observed in El Nino and the Quasi-Biennial Oscillation were also identified. We conclude that the solar signal is probably present in the minimum extreme temperature record of the central part of Mexico.

Geoeffectiveness of the heliospheric current sheet

In the present paper we study the distribution of storm sudden commencements (sscs) and their associated geomagnetic activity with respect to the Earths crossing of the heliospheric current sheet. We found that most ssc events occur when the Earth is close to the current sheet. Also, we found that the geoeffectiveness of sscs seems to be independent of their position with respect to the current sheet. Our results seem to suggest that coronal mass ejections, assumed as the sources of most sscs, should follow the behavior of sscs along the solar cycle, i.e., the shock-producing ones should present a bimodal distribution.

Coronal holes as sources of large-scale solar wind disturbances and geomagnetic perturbations

In this paper, the authors present theoretical and observational evidence that low-latitude unstable coronal holes may be, in some circumstances, the source of widespread interplanetary shocks and of the consequent geomagnetic perturbations. They also reanalyze here the shock events tracked by interplanetary scintillation in the study of Hewish and Bravo (1986), which produced the sudden commencement of a geomagnetic storm, in terms of an altenative flare, disappearing filament, or coronal hole eruption origin. The main conclusion of this study is that there are good reasons to consider coronal holes as sources of shock type interplanetary disturbances and that more effort should be made to monitor coronal holes continuously in search of evidence of large short-time changes in their structure and physical conditions.

Spectral analysis results for sudden storm commencements 1868–1996

Abstract We perform a power spectrum analysis of the occurrence of sudden storm commencements for solar cycles 11 to 22. Studying in particular peaks longer than the 11 years solar cycle, we found a significant periodicity at around 20 to 30 years, which seems to be unstable. Similar three-cycle unstable peaks are observed in other solar and geomagnetic phenomena such as sunspots, auroral occurrence and the Ap index. The origin of such a periodicity in the occurrence of storm commencements is unknown at present. Due to its unstable behaviour it is not possible to predict when it will appear in the SSCs data.

Can the low-Activity sun Become Dimmer at Maximum?

After analysing the ratio of sunspot to facular areas along the cycle for solar cycles 12 to 20 we propose two possibilities. One indicates a non-linear behaviour for low-activity cycles and a closer-to-linear behaviour for high-activity cycles, the other one presents a non-linear behaviour for both low- and high-activity cycles and a closer-to-linear behaviour for moderate-activity cycles. Furthermore, we also find within the cycle that during low-activity cycles the Sun becomes brighter as their magnetic activity level increases while for high-activity cycles the opposite occurs, in agreement with previous studies of solar-type stars; another possibility, however, is that when evolving from minimum to maximum both the low- and high-activity Sun may become fainter while the moderate-activity Sun brightens.

Geomagnetic response of the Earth to crossings of the heliospheric current sheet

We performed a superposed epoch analysis of the aa index of geomagnetic activity with respect to the Earths crossing of the heliospheric current sheet during periods when quiet and fluctuating activities are detected from 1976 to 1985. We found a statistically significant increase of the geomagnetic activity during boundary crossings when the polarity of the interplanetary magnetic field changes from minus to plus.

The intense solar activity of March 1989 as a precursor for the occurrence of an ENSO by the end of 1989

We studied the occurrence of ENSO (El Niño-Southern Oscillation) events in relation to solar activity conditions and we found that the events are distributed in two populations, one that could be associated with low latitude coronal holes which occur along the ascending phase of the cycle and the other one which could be related to polar holes present at the end of the descending phase. As highly anomalous solar activity, in terms of flares and coronal holes, occurred in March of this year, we propose that an ENSO event is likely to occur by the end of the year.

The role of flares, CMEs and CME shocks in the generation of solar energetic proton events

We examined solar energetic proton (SEP) events associated with intense Hα flares. We located these flares on the solar disk and obtained their distribution in heliographic longitude as well as their angular distance distribution with respect to the neutral lines corresponding to the heliospheric current sheet at 2.5R⊙. We found that the SEP-associated Hα flares tend to occur in active regions at the feet of those helmet streamers which form the heliomagnetic equator and are related to coronal mass ejections (CMEs) and CME shocks. We discuss the possible role of flares, CMEs and CME shocks in generating SEPs.