M. Storini

The longitudinal asymmetry of the interplanetary perturbation producing Forbush decreases

SummaryIt is well known that type-IV solar flares occurring between 0° and 40° East of the central meridian can produce the largest Forbush decreases observed at the Earth,i.e. inside the Forbush decrease modulated region the maximum cosmic-ray modulation is observed in the West (∼20° W) of the flare’s meridian plane. A flare-produced perturbation asymmetric in longitude is required in order to explain this asymmetry. The analysis of the two-step Forbush decreases shows that the first step of the decrease is symmetric with respect to the flare’s meridian plane and it is due to the shock wave which is observed ahead of the interplanetary perturbation; on the contrary the second step is strongly asymmetric, the maximum amplitude being observed at ∼30° W of the flare’s meridian plane, and it seems to be due to the magnetic perturbation following the shock; this perturbation is an irregular and compressed magnetic field, the maximum magnetic-field magnitude is found to be placed West of the flare’s meridian plane. The total Forbush decrease amplitude is correlated (correlation coefficient 0.96) with a perturbation parameter which accounts for the «strength» of the shock and of the following magnetic perturbation. The first step of the decrease is correlated (correlation coefficient 0.88) with the shock «strength».RiassuntoÈ ben noto che i brillamenti solari accompagnati da radioemissione di tipo IV nell’intervallo 0°–40° est possono produrre le maggiori diminuzioni di Forbush osservate alla terra; cioè, dentro la regione interplanetaria interessata dalla dimizuzione di Forbush la massima modulazione dei raggi cosmici è osservata a circa 20° ovest dal piano meridiano del brillamento solare. Per spiegare questa asimmetria si richiede l’esistenza di una perturbazione interplanetaria generata da brillamento che sia di tipo asimmetrico. L’analisi delle diminuzioni di Forbush «a due gradini» mostra che la prima diminuzione ha una dipendenza simmetrica rispetto al piano meridiano del brillamento ed è dovuta all’onda d’urto che è osservata all’inizio della perturbazione stessa. La seconda diminuzione, al contrario, è fortemente asimmetrica poiché la massima ampiezza è osservata a circa 30° ovest del piano meridiano del brillamento; essa sembra essere dovuta alla perturbazione magnetica (caratterizzata dal campo magnetico compresso e irregolare) che segue l’onda d’urto; la massima ampiezza del campo magnetico si trova a ovest del piano meridiano del brillamento. L’ampiezza totale delle diminuzioni di Forbush è correlata (coefficiente di correlazione 0.96) con un parametro empirico della perturbazione che tiene conto dell’importanza dell’onda d’urto e della perturbazione magnetica che segue, mentre il primo «gradino» della diminuzione è correlato (coefficiente di correlazione 0.88) soltanto con il parametro dell’onda d’urto.РрзюмеХорошо известно, что солнечные вспышки типа IV, возникающие в интервале между 0° и 40° к востоку от центрального меридиана, могут порождать большие убывания форбуша, наблюдаемые на Земле, т.е. внутри модулированной области убывания форбуша наблюдается максимальная модуляция космических лучей около 20° к западу от меридианной плоскости солнечной вспышки. Для объяснения этой асимметрии требуется существование межпланетного возмущения, образованного вспышкой, которое является асимметричным по долготе. Анализ двух-ступенчатых убываний форбуша показывает, что первая ступень убывания является симметричной относительно меридианной плоскости вспышки и обусловлена ударной волной, которая наблюдается перед межпланетным возмущением; напротив, вторая ступень является сильно асимметричной, максимум интенсивности наблюдается около 30° к западу от меридианной плоскости вспымки, и, по-видимому, является результатом магнитного возмущения после ударной волны; это возмущение является иррегулярным и вызывает сжатие магнитного поля. Максимум амплитуды магнитного поля расположен к западу от меридианной плоскости вспышки. Полная амплитуда убывания форбуша коррелирует (коэффициент корреляции 0.96) с параметром возмущения, который характеризует «интенсивность» ударной волны и последующее возмущение магнитного поля. Первая ступень убывания Форбуша коррелирует (коэффициент корреляции 0.88) только с «интенсивностью» ударной волны.

Interplanetary disturbances during Forbush decreases

SummaryThe interplanetary perturbations associated with Forbush decreases (FDs) during the period 1965–1974 are studied. The systematic appearance of the results obtained confirms once again that type-IV solar flares are the origin of these perturbations. The front perturbation is a wide (∼100° in longitude) driven shock followed by a magnetic blob and a high-speed plasma cloud of about 0.5 A.U. average radial dimension at the Earth orbit, emitted in a short time interval (<15 h) immediately following the beginning of the type-IV burst. The FD-modulated region is found to be included between two boundary streams which rotate with the Sun; therefore the FD can be observed also in the interplanetary regions which were not swept by the front perturbation. The FD amplitude appears to be correlated with an empiric parameter which caters for the importance of the shock and magnetic blob at the front perturbation.RiassuntoLe perturbazioni interplanetarie associate alle diminuzioni di Forbush (FDs) sono state studiate per il periodo 1965–1974. L’insieme dei risultati ottenuti conferma che i brillamenti solari accompagnati da radioemissione di tipo IV sono all’origine di queste perturbazioni. II fronte della perturbazione è un’onda d’urto di estensione ∼100° in longitudine seguita da campo magnetico esaltato («magnetic blob») e da una nuvola di plasma ad alta velocità, di dimensione media radiale ∼0.5 A.U. all’orbita della Terra, emessa per un intervallo di tempo breve (,15 ore) dopo l’inizio della radioemissione di tipo IV. Si trova che la regione modulata della FD è limitata da due fasci di vento solare veloce che coruotano con il Sole; la FD può essere perciò anche osservata nelle regioni di spazio interplanetario che non sono state investite direttamente dal fronte della perturbazione. L’ampiezza della FD appare correlata con un parametro empirico che tiene conto dell’importanza dell’onda d’urto e del «magnetic blob» presenti al fronte della perturbazione.РезюмеИсследуются межпланетные возмущения, связанные с убываниями Форбуша в течение периода 1965–1974 г.г. Получается систематическое подтверждение того факта, что солнечные вспышки типа IV связаны с этими возмущениями. Фронт возмущения представляет широкую (∼100° по долготе) ударную волну, за которой следует магнитое «пятно» и плазменное облако со средним радиальным размером 0.5 а.е.д. на орбите Земли, испускаемые в течение короткого временного интервала (<15 часов) непосредственно за началом вспышки типа IV. Область, модулированная убываниями Форбуша, заключена между двумя граничными потоками, которые вращаются вокруг Солнца. Таким образом, убывания Форбуша могут наблюдаться также в межпланетных областях, через которые не проходит фронт возмущения. Оказывается, что амплтуда убываний Форбуша коррелирует с змпирическим параметром, который определяет значимость ударной волны и магнитного «пятна» на возмущение Форбуша.

Aspects of the long-term cosmic-ray modulation

To investigate the long-term modulation of galactic cosmic rays at the ground-based detector energies, the monthly values of the neutron monitor (Climax, Mt. Washington, Deep River, and Huancayo) and ionization chamber (Cheltenham/Fredericksburg, Huancayo, and Yakutsk) intensities have been correlated with the sunspot numbers (used as a proxy index for transient solar activity) for each phase of sunspot cycles 18 to 22. Systematic differences are found for results concerning odd and even sunspot cycles. During odd cycles (19 and 21) the onset time of cosmic-ray modulation is delayed when compared with the onset time of the sunspot cycle, while they are more similar during even (18, 20, and 22) cycles. Checking the green corona data, on a half-year basis, we found typical heliolatitudinal differences during ascending phases of consecutive sunspot cycles. This finding suggests a significant role of the latitudinal coronal behaviour in the heliospherical dynamics during a Hale cycle. Such effectiveness concerns not only the transient interplanetary perturbations but also the recurrent ones. In fact, when lag between cosmic-ray data and sunspot numbers is considered, the anticorrelation between both parameters is very high (correlation coefficient |r| > 0.9) for all the phases considered, except for the declining ones of cycles 20 and 21, when high-speed solar wind streams coming from coronal holes affect the cosmic-ray propagation, and theRz parameter is no longer the right proxy index for solar-induced effects in the interplanetary medium.

Galactic cosmic-ray modulation and solar-terrestrial relationships

Representative examples of the Italian scientific undertaking on galactic cosmic-ray modulation within the GIFCO projects are given. Also current ideas from the international community on long-term effects are discussed.

TWO COMPONENTS OF COSMIC-RAY MODULATION

The long-term modulation of galactic cosmic rays is investigated from 1957 up to 1992 analysing the dynamic and the quasi-stationary components, separately. It has been found that the dynamic component is characterized by the presence of two peaks at the maximum phase of each solar activity cycle. We infer that the time interval between the two peaks corresponds to a period (well-related to the polar heliomagnetic reversal) in which somewhat decreased activity occurs for intense and long-lasting solar events. In fact, a contemporary dip in the magnetic energy released from the Sun was observed, in agreement with the suggested double maximum displayed by the basic features of the 11-year solar-activity cycle (Gnevyshev, 1977, and references therein). Moreover, the dynamic component of cosmic-ray modulation often shows a multi-structured profile in both peaks of activity, fairly well-connected with the pattern of the green corona brightness. On the other hand, analysing the quasi-stationary long-term trend of cosmic-ray intensity we pick out a good relationship between periods of enhanced cosmic-ray modulation and the area expansion of coronal intensity levels. The relevance of our results for solar-terrestrial forecasting is underlined.

Solar wind in the near-Earth interplanetary space: 1964–1987 revisited

SummarySolar-wind data, as obtained in the near-Earth space, have been re-examined for the period 1964 to 1987. For each month, the maximum and minimum hourly values of the solar-wind bulk speed together with those for the interplanetary magnetic-field intensity have been selected. They are reported in a tabular form (available as readable ASCII text files on DOS-formatted floppy disks) to ease off further studies in the framework of the Solar-Terrestrial System.

North-South anisotropy during the quasi-stationary modulation of galactic cosmic rays

SummaryHigh-speed solar-wind streams coming from coronal holes produce, in the interplanetary space, a quasi-stationary modulation of galactic cosmic rays. The Rome cosmic-ray group has analysed this modulation taking into account not only the charged-particle isotropic intensity but also the North-South anisotropy (ANS), derived from ground-based measurements. The obtained results for the period October 1964–December 1987 are summarized. Data from the Bartels solar rotation no. 2034 (May 23–June 18, 1982) are used to illustrate theANS phenomenology.

Cosmic-ray measurements in Antarctica during the international Solar-Terrestrial Energy Program

SummaryConcomitant with the development of the international collaboration on STEP (Solar-Terrestrial Energy Program: 1990–1995) the Universities of Chile and ofMagallanes have proposed to the Chilean Antarctic Institute (INACH) an experimental research to study the cosmic radiation on the Austral ground. The project, which has been approved to start in 1991, consists in the installation of a 6-NM-64 on the T.te Marsh Base (geographic latitude: ∼62 South, longitude: ∼59 West, King George Island). The scientific objectives related to the project are briefly described.

Cosmic-ray intensity increase observed on September 29, 1989: the Chilean neutron monitor data

SummaryData obtained in Chile with the standard neutron monitor (Santiago: 6 NM-64; geomagnetic cut-off rigidity: about 11 GV) are reported for the ground-level solar cosmic-ray event registered on September 29, 1989. Possible studies connected with such a rare event are suggested to understand even more solar-interplanetary-terrestrial relations.