The strength of galactic cosmic rays (GCRs) reaching Earth varies in reaction to changes in the Sun’s magnetic field, which affects how cosmic ray particles like electrons and protons move through space. More GCRs have been seen to reach Earth during times of low solar activity, such as the solar minimum, than during strong solar activity. “Solar modulation” refers to this inverse relationship between the GCR-flux and solar activity.
It has recently been discovered that this impact depends on the charge sign of their constituent particles thanks to measurements made with the CALorimetric Electron Telescope (CALET) over six years. The drift model of cosmic ray transport in the heliosphere is supported by the tilt angle of the heliospheric current sheet, which has a greater impact on the count rates of electrons than protons.
The tilt angle of the heliospheric current sheet (HCS), a spiral surface separating the direction of oppositely directed magnetic field lines coming from the poles of the Sun, influences the strength of GCRs on Earth. The intensity of cosmic rays on Earth decreases as the HCS’s tilt angle increases.Â
The drift model of GCR transport in the heliosphere suggests that the negatively charged electrons in GCRs prefer to travel along the HCS to reach Earth if the magnetic field is pointed away from the Sun in the northern hemisphere and towards the Sun in the southern hemisphere. Contrarily, positively charged protons transit via the HCS to reach Earth from the heliospheric polar zone, indicating that GCR electrons are more sensitive to solar modulation than protons.
The fluxes of positively and negatively charged GCR particles during the solar cycle can be observed differently in previous observations of cosmic ray particles made aboard space balloons and in space experiments. Still, it is unknown whether the particle charge affects the anti-correlation between GCR intensity and the tilt angle of the HCS.
Now, scientists have discovered that this anti-correlation is, in fact, more pronounced for electrons than for protons in recent observations of GCR-charged particles made with the CALorimetric Electron Telescope (CALET) onboard the International Space Station’s “Kibo” Exposed Facility (EF) over six years.
Using CALET, scientists successfully observed a charge-sign-dependent solar modulation of GCR over six years.
Over a six-year period from 2015 to 2021, which coincided with the conclusion of solar cycle 24 and the start of solar cycle 25, the researchers examined more than 0.77 million electrons and 1.26 million protons gathered in around 196 and 197 hours, respectively.
The results showed that electron and proton count rates were low but gradually rising during the low activity condition of the Sun near the conclusion of solar cycle 24, which was characterized by a decrease in the number of sunspots and HCS tilt angle. With the start of solar cycle 25, this pattern persisted, and six months into the cycle, in December 2019, it peaked in terms of electron count rate.
Thereafter when the Sun’s activity and the HCS tilt angle increased, both the electron and proton count rates gradually decreased. The findings also revealed that during this time, there was a substantially greater variation in the count rates of electrons than protons, indicating that electrons are more vulnerable to the effects of solar modulation than protons are, as predicted by the drift model.
Associate Professor Yosui Akaike of the Waseda Research Institute for Science and Engineering (RISE) at Waseda University said, “This is a clear sign of the drift effect dominating the long-term solar modulation of GCR observed with a single detector.”
Journal Reference:
- O. Adriani et al. (CALET Collaboration). Charge-Sign Dependent Cosmic-Ray Modulation Observed with the Calorimetric Electron Telescope on the International Space Station. Physical Review Letters. DOI: 10.1103/PhysRevLett.130.211001