Cosmic Ray Decreases Research Articles

Interpretation of Short-Term Solar Variability Effects in the Troposphere

Among possible external forcing agents for tropospheric response to solar variability, the variation of MeV-GeV particle fluxes, resulting in changes in stratosphere-troposphere ionization and electric fields, is a plausible candidate. The MeV-GeV particle hypothesis fits reported correlations of geopotential heights, temperatures and vorticity area index (VAT) on the one hand with solar proton events, high-speed solar wind plasma streams, magnetic storms and Forbush decreases of galactic cosmic rays on the other hand. Both solar wind magnetic structures and changes in the terrestrial magnetosphere modulate the incoming particle fluxes. The day-to-day correlations are strongest in winter.Mechanisms linking MeV-GeV flux changes with tropospheric changes are speculative. Among a number of processes to be investigated, one not previously considered involves the electrofreezing of supercooled water droplets in high clouds of cyclones; their growth by the Wegener-Bergeron instability; sedimentation to lower level supercooled clouds where they enhance freezing and latent heat release; the intensification of convection; and in warm core winter cyclones the extraction of energy from the baroclinic instability to further intensify the cyclone; leading to changes in the general circulation. On a longer time scale changes in water/ice ratio in cirrus may lead to changes in cloud radiative forcing and also to changes in the general circulation.

Two examples of strong magnetic clouds interacting with the co-rotating stream

Two intense magnetic clouds interacted with the same corotating high-speed solar wind stream in two successive rotations in the fall of 1978. In the August 27 event, the cloud was overtaken by the stream, the front half of the cloud still retained the basic properties of a simple intense cloud while the back half was the interaction region with a two-step velocity profile. In the September 25 event, the cloud was overtaking the same stream, its front half was now the interaction region while the back half had a rather more complex structure than a simple, non-interacting cloud. In the second event, the bulk velocity reached a very high peak value more than 900 km/s in the interaction region, and the Alfvénic speed in the cloud body also reached a high value of more than 600 km/s, which was the local solar wind speed. These two events show further the importance of an enhancement of the Al fvénic speed in accelerating the cloud. The properties of the geomagnetic storm and cosmic ray decrease caused by the clouds are also briefly discussed with reference to the Kp category of geomagnetic storms proposed by the author.

Magnetic clouds, geomagnetic disturbances, and cosmic ray decreases

Nineteen magnetic clouds are identified in the years from 1978 through 1982 and studied using superposed epoch analysis. The magnetic field intensity, proton density, and proton temperature are enhanced ahead of magnetic clouds that are preceded by a shock, while strong magnetic field intensity and low proton temperature are observed within the clouds. A relatively large (∼2.5%) decrease in cosmic ray intensity is associated with magnetic clouds that are preceded by a shock, perhaps caused by the turbulent sheath behind an interplanetary shock ahead of the magnetic cloud, whereas only a small (∼0.5%) decrease in intensity is associated with the magnetic cloud itself. Magnetic clouds can produce geomagnetic activity with a decrease in Dst index of the order of 100 γ. The magnitude of the change in the Dst index for the case when southward fields arrive first is comparable to that for the case when northward fields arrive first, and the phase is such that geomagnetic activity is associated with southward fields.

Tracking of flare-generated interplanetary shock waves, responsible for Forbush decreases, by observing type-II radio bursts

New possibilities are pointed out of a more accurate identification of solar flares, responsible for Forbush decreases of galactic cosmic rays, caused by flare-generated interplanetary shock waves, based on tracking type-II radio bursts. Nine solar flares, observed between September 1978 and December 1979, generated shock waves which produced intensive type-II radio bursts observed in the hectometric and kilometric wave range by the ISEE-3 satellite. All the type-II shocks were responsible for Forbush decreases. Since the observation of type-II radio bursts enables us to follow the propagation of shock waves practically from the place where they are generated deep in the solar corona, through interplanetary space as far as the Earth, these observations provide the possibility of exact identification of Forbush decreases with the corresponding flares. These observations have also supplied a further dimension for analysing interplanetary disturbances and forecasting Forbush decreases.

Solar Wind Data and Ionospheric Potential

Solar wind velocity increases are associated with galactic cosmic-ray decreases. A comment by Barouch about the possible correlation between these observations is followed by a discussion by Markson. (GHT)

Solar Wind Data and Ionospheric Potential

Solar wind velocity increases are associated with galactic cosmic-ray decreases. A comment by Barouch about the possible correlation between these observations is followed by a discussion by Markson. (GHT)

On the charge state of the anomalous oxygen component

We report our observations of a phase lag between the intensity variations of high energy galactic cosmic rays and the quiet time oxygen flux in the energy region 7.6‐24 MeV/nucleon during the time period 1974‐1975. Phase lag or hysteresis effects have been observed during the 11‐year solar cycle for low energy galactic cosmic rays and several authors have shown that this hysteresis effect is a function of the velocity and rigidity (and therefore of the ionic charge) of the particles. Assuming a similar rigidity dependence of the phase lag during the transient intensity decrease of both galactic cosmic rays and the anomalous oxygen component in the time period 1974‐1975, and comparing the phase lag as observed for protons and helium during the same period with our oxygen measurements, we derive an upper limit of ∼4 for the ionic charge of the anomalous oxygen component.

The cosmic-ray storm of February 15, 1978

During the Forbush decrease of February 14, 1978 a huge cosmic-ray decrease lasting only ≈10 hours was observed by the neutron monitor stations. A detailed analysis, performed on data of numerous neutron monitors, shows that this event is a bidirectional anisotropy with a cosmic-ray density gradient rising from a plane tilted ≈30° with respect to the Earth equator; the interplanetary magnetic field should be perpendicular to that plane.

Solar-cycle evolution of the coronal general magnetic field of 1959?1974 and the synchronous variation of high-speed solar wind streams and galactic cosmic rays

The simultaneous enhancement or subsidence of both the high-speed solar wind streams and the galactic cosmic rays in the minimum or the maximum phase of the solar cycle are interpreted in a unified manner by the concept of geometrical evolution of the general magnetic field of the corona-heliomagnetosphere system. The coronal general magnetic field evolves from an open dipole-like configuration in the minimum phase to a closed configuration with many loop-like formations in the maximum phase of the solar cycle. This concept, developed in a theoretical solar-cycle model driven by the dynamo action of the global convection, is examined and found to be valid by studying the evolution of the coronal general magnetic field calculated from the observed surface general magnetic field of 1959–1974. It is also found that the energy density of the poloidal component of the general surface field, from which the coronal field originates, attained a maximum in the maximum phase and showed a evolution with virtually no phase delay with respect to that of the toroidal component of the field, to which the sunspot activity is related. The subsidence of the high-speed solar wind in the maximum phase is understood as a braking of the solar wind streams by the tightly closed and strong coronal field lines in the lower corona in the maximum phase. The field lines of the heliomagnetosphere, which originate from the coronal field lines drawn by the solar wind, are inferred to be also more tightly closed at the heliopause in the maximum phase than in the minimum phase. The decrease of the galactic cosmic rays in the maximum phase (known as the Forbush's negative correlation between the galactic cosmic ray intensity and the solar activity or the Forbush solar-cycle modulation of the galactic cosmic rays) is interpreted as a braking of the cosmic rays by the closed magnetic field lines at the heliopause. The observed phase lag (approximately one year) of the galactic cosmic ray modulation with respect to the evolution of the solar cycle, and the observed absence of the gradient of the total cosmic ray intensity between 1 AU and 8 AU, are discussed to support this view of the cosmic ray modulation at the remote heliopause, and reject other hypotheses to explain the phenomenon in terms of the magnetic irregularities of various kinds carried by the solar wind: The short-term Forbush decrease at a time of a flare shows that the magnetic irregularities can react on the cosmic rays relatively near the Sun if they even played a dominant role in the long-term modulation. The concept of the general magnetic field of the corona and the surface is also used to understand the basic nature of the surface field itself, by comparing the geometry of the calculated coronal field lines with the eclipse photographs of the corona, and by discussing, in the context of the coronal general magnetic field associated with the solar cycle, the process of the emergence of the coronal field lines from the interior and the formation of the transequatorial arches and loops connecting the two hemispheres in the corona.

The origin of transient cosmic ray intensity variations

A new approach to determining the solar progenitor of transient cosmic ray intensity variations has revealed that in a statistical sense, solar flares, heretofore regarded as the predominant source of the modulation, actually do not precede the reduction in flux observed at earth. Superposed epoch analysis of the cosmic ray data with respect to the time of occurrence of all 379 solar flares of importance (Imp) ≥ 2 observed during solar cycle 20 (1964–1974 inclusive) shows that the onset of a decrease in the composite nucleonic intensity at polar stations occurs prior to the zero day (i.e., time of the flare) well before the arrival in the vicinity of earth of the associated solar plasma. The statistical significance of this result is confirmed by comparing the pooled variance determined from Chree analysis of an equal number of random epochs with that of the curve representing the flare epochs. Subdivision of the latter into three groups according to the heliographic longitude of the flares shows that whereas eastern flares might be associated with cosmic ray decreases, central (30° to −30°) and western flares cannot be thus related. A similar analysis of all flares of Imp ≥ 2 that occurred in a selected set of 24 extraordinary flare-rich active centers during 1964–1974 confirms these results and shows that the observed cosmic ray intensity decrease is, in fact, associated with the central meridian passage (±1 day) of the active regions. Thus earlier conclusions concerning relationships between the heliolongitude of flares and their apparent effectiveness in producing Forbush decreases require reevaluation. The specific feature associated with solar active centers that is actually the principal source of transient modulations remains to be identified.

Two classes of cosmic ray decrease

In a study of galactic cosmic ray intensity variations it had been found that there are two distinct classes of transient intensity decrease, which have been called type I and type II. Type I events are flare-associated Forbush decreases. Type II events appear to be associated with 27-day recurrences. It is suggested, on the basis of several characteristics of the decreases, that type II events are simply the subsequent evolution of type I events or quasi-stationary 'corotating' structures loosely associated with active regions.

Cosmic-ray decreases and the occurrence of solar flares

Results are presented of a more extensive study of the relationship between the occurrence of cosmic-ray decreases and solar flares. In particular, it is considered whether it is possible to relate all large cosmic-ray decreases to flare-producing regions on the sun, and whether there is any compelling evidence relating the time of a large flare and the occurrence of a decrease to substantiate the long-standing hypothesis that at least some large cosmic-ray decreases are due to the effects of a single energetic flare. It is concluded that existing flare evidence significantly favors the hypothesis that all large cosmic-ray decreases can be accounted for by interplanetary structure associated with active flare-producing regions.

Trains of diurnal waves in the muonic component at 70 MWE during cosmic-ray decreases

Trains of diurnal waves in the muonic component at 70 MWE have been found during the preliminary phases of some cosmic-ray decreases. A predecrease the day immediately preceding a Forbush decrease has repeatedly been reported from neutron monitor observations. Our analysis indicated that underground data reveal this effect earlier and for a longer period. This is consistent with the interpretation of a Forbush decay as the result of the advance of a shock front.

Effects associated with the sector boundary crossing on July 8, 1966

Thirty hours after the proton flare of July 7, 1966, the earth and nearby satellites crossed a sector boundary of the interplanetary magnetic field. This occurred before the flare-associated shockwave arrived at the earth, so that the space was filled with energetic particles ejected from the flare. Satellite measurements have shown that in such a case <20 MeV protons are stored within the range of the sector boundary and with decreasing energy the particles tend to accumulate towards and behind the Eastern boundary limit; low-energy particles, such as <50 keV electrons, are stored exclusively behind this Eastern limit. The boundary crossing caused a short-lived geomagnetic disturbance, a PCA enhancement in lower latitudes, a two-phase ionospheric disturbance, and a transient cosmic-ray decrease on neutron monitors looking westward immediately after the sector boundary passed the earth. The storage of low-energy particles on the Eastern side of the boundary may indicate a preference of the transverse diffusion for the Westward direction in interplanetary space.

Change of the main characteristics of cosmic-ray variations and interplanetary space with the cycle of solar activity

Changes of the characteristics of the solar-diurnal variation and Forbush decreases of cosmic rays with the solar activity cycle are analyzed. It is shown that the energy spectrum of the solar-diurnal variation is well described by the Krymsky diffusion mechanism spectrum with b parameters of 59, 40, and 30 BeV for high, middle, and low solar activity levels, respectively. The Forbush-decrease spectrum is described by a power law with exponents −0.8 and −0.6 during high and low solar activity. These changes are interpreted to be the result of a decrease of solar wind velocity and a decrease of the mean strength of the interplanetary magnetic field with a decrease in solar activity.

On the existence of a phase anticipation in trains of diurnal oscillations of cosmic rays

From a previous study of the association of diurnal oscillations and Forbush decreases of cosmic rays, an hypothesis was put forward that the Forbush decreases were complementary to diurnal oscillations. Inspecting the hourly statistics from meson scintillation monitors, it is concluded that many times the diurnal oscillations anticipate their phases by one day.

Sudden Expansion of the Corona Following a Large Solar Flare and the Attendant Magnetic Field and Cosmic-Ray Effects.

It is shown from the hydrodynamic equations that the 4 x 10/sup 6/ deg K temperatures observed in the solar corona after a large solar flare can produce a hydrodynamic blast wave, moving out through interplanetary space with velocities of 1500 km/sec and densities from a few times 102 to 105/cm/sup 3/ at the orbit of earth. It is suggested that this hydrodynamic explosion of the enhanced corona is the accelerating mechanism for the plasma clouds or enhanced solar corpuscular emission responsible for the geomagnetic storm, the cosmic-ray decrease, the low-latitude aurorae, etc., observed at earth a day or two following large solar flares on the visible hemisphere of the sue. The interplanetary magnetic field is computed in the blast wave, assuming an initial quiet-day solar wind of a few hundred km/sec ahead of the wave. The spiral field of the quiet-day wind is sheared by the blast wave, and its density, of the order of initial quiet-day solar wind of a few hundred km/sec ahead of the wave. The spiral field of the quiet-day wind is sheared by the blast wave, and its density, of the order of 2 x 10/sup -5/ gauss at the orbit of earth, maymore » be increased to 5 x 10/sup -4/ gauss or more for a period of several hours. The effect of the outward-sweeping magnetic shear in the blast wave is to decrease the cosmic-ray intensity, by as much as 40%, in the inner solar system behind the blast wave. The onset of the decrease has a characteristic time of several hours, and the relaxation many hours or days. The energy dependence of the decrease may go inversely with particle rigidity, or it may be flatter, depending on the details of the magnetic configuration in and around the blast wave. Thus the cosmic-ray decrease produced by the blast wave is identical with the Forbush-type cosmic-ray decrease observed in association with enhanced solar corpuscular radiation. (auth)« less

Auroral x-rays, cosmic rays, and related phenomena during the storm of February 10-11, 1958

Balloon observations were made during the auroral storm of February 10- 11, 1958, at Minneapolis. Strong x-ray bursts in two groups were detected. The groups appeared coincident with two large magnetic bays, with strong radio noise absorption, and with the passage across the zenith of a very large amount of auroral luminosity. From the x-ray intensity and measured energies, an electron current of 0.6 x 10/sup 6/ electrons /cm/sup 2// scc was present. These electrons ionizing the upper D layer accounted for the increased cosmic noise absorption. The x-rays themselves carried 1000 times less energy than the electrons and could not provide sufficient ionization for the observed radio absorption. Visual auroral fornis during this storm are reported to have lower borders at thc 200 to 300 km level. There is thus a difficulty in bringing the electrons to the D layer without ani accompanying visible aurora. A cosmic-ray decrease accompanied the storm and was observed to be from 4 to 6% at sea level, 21% in the balloon altitude ionization, and 15% in total energy influx at 55 deg geomagnetic latitude. Compared with the great intensity of the magnetic and auroral phenomena in this storm, the cosmic-ray modulation was notmore » exceptionally large. (auth)« less

Solar activity and transient decreases in cosmic-ray intensity

The world-wide character of the intensity changes in meson and nucleon components is shown by a study of data from Ottawa, Churchill, and Resolute during the period October 1956 to December 1957. Further discussion is essentially restricted to the mean nucleonic component derived from the three stations. The investigation reveals an association between transient decreases in cosmic-ray intensity and the central meridian passage of active solar regions. On an average, the greater the activity rating of the regions, the larger is the cosmic-ray decrease. It is not possible, however, to discuss the relation specifically in terms of the characteristics of the regions, such as flares, sunspots, and the like. Further support for the relation comes from observation of geomagnetic data. Attention is drawn to the similarity between small as well as large transient decreases in intensity with regard to presence or absence of recurrence tendency. The study supports the view that both the 27-day variation and the Forbush events differ only in degree and could therefore be attributed to the same mechanism. It is possible to attribute the cosmic-ray decreases to beams of ionized rarified gas emitted from the sun and differing considerably in their characteristics.

Rapid decrease of cosmic ray intensity

Two observers at Cambridge, Mass., using a very high rate meson monitor near sea level reported a small cosmic-ray decrease, beginning at O100 UT on February 11, 1958, some 20 to 25 min before the sudden commencement of a magnetic storm, and lasting for about 1.75 hours. Readings obtained from each of two neutron monitors in the Deep River station, also near sea level, appear to confirm this change, although the counting rates are very much smaller. Both the meson and neutron counter data are given and discussed. A description is given of the neutron monitors, and possible advantages of monitoring cosmic radiation by neutron counting are considered. (A.C.)