Leif Svalgaard at AGU on the Current Solar Cycle: ‘None of us alive have ever seen such a weak cycle’

WUWT’s resident solar expert Dr. Leif Svalgaard (and others) says  ‘None of us alive have ever seen such a weak cycle’  and the panel he was on talk about the current state of our solar cycle at the AGU Fall Meeting.

Here is Dr. Svalgaard’s current SSN plot:

Watch the video, Leif is on the left hand side.

At this year’s Fall Meeting of American Geophysical Union, held in San Francisco that I attended, prominent solar scientists made a presentation on weak Solar Cycle 24 and its consequences. They included:

• Nat Gopalswamy, astrophysicist, Solar Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland
• Leif Svalgaard, senior research scientist, W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, California
• Marty Mlynczak, senior research scientist, Climate Science Branch, NASA Langley Research Center, Hampton, Virginia
• Joe Giacalone, professor and associate director, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona

They agreed that the current solar cycle is on track to be the weakest in 100 years and that is an unprecedented opportunity for studying the Sun during this period. While the weak solar cycle trend is not new for the Sun, it is new and interesting for scientists who observe and measure it today with modern instruments and methods.

In this panel, scientists examined the current solar cycle in relation to past cycles and discuss the consequences of the weak solar cycle on the various layers regions between the Sun and Earth, including implications for space weather, atmosphere and climate.

Here is part of the press release package:

Solar signatures and Heliospheric Consequences of the Weak Activity Cycle 24

Nat Gopalswamy, NASA Goddard Space Flight Center, Greenbelt, MD 20771,

The Sun in the middle of its activity maximum that is relatively weak. The maximum phase ended in the northern hemisphere of the Sun and began about a year ago in the south.

The weak activity of cycle 24 is thought to be due to the weak polar magnetic field in cycle 23. If this trend continues for the next couple of cycles, the Sun may be heading for a global minimum.

Whether global minimum or not, the weak solar cycle has resulted in milder space weather: there are not many large geomagnetic storms and the energetic particle events are also generally of lower intensity. The milder space weather also reduces the drag on satellites and it is easy to keep them in orbit. On the other hand the space debris also have longer life, posing increased collision threat to operating satellites.

The weak solar activity in terms of the sunspot number did not quite translate into the CME rate itself. The CME occurrence rate in cycles 24 and 23 are comparable in the maximum phase. Then how do we understand the mild space weather in cycle 24?

A clue to the reason for milder space weather came from the fact that all CMEs that produced particle events are halo CMEs in cycle 24, compared to about 70% in cycle 23. Halo CMEs originate from close to the disk center and expand rapidly and give the appearance of surrounding the Sun. There must be something different about the size of the CMEs in SC 24.

Gopalswamy and co-­‐workers examined the relation between CME width and speed and found that the cycle 24 CMEs are wider than the cycle 23 ones for a given speed. For energetic CMEs (speed exceeding 1000 km/s), the width is higher by about 40%.

When they examined the total pressure (magnetic pressure + plasma pressure) in the heliosphere from measurements made by spacecraft such as ACE and Wind, they found that the pressure decreased by an astonishing 40% in cycle 24. From this they inferred that the pressure must drop by a similar amount near the Sun. CMEs released into this low-­‐pressure medium, expand more than usual, resulting in weaker fields, and hence weaker geomagnetic storms. The magnetic field strength in CMEs decides the intensity of geomagnetic storms.

As far the particle radiation, the situation is a bit more complicated. The reduced total pressure means a slight increase in the Alfven speed in the heliosphere. The Alfven speed is the characteristic speed of the medium. A CME needs to be faster than the Alfven speed to drive a shock that accelerates particles.

Therefore, it is slightly easier for the cycle 24 CMEs to drive shocks. However, the shocks are propagating through a medium of reduced magnetic field, which is known to be less conducive for accelerating particles to high energies. This means the number of particle events is not very low, but the events are generally of lower intensity and energy.

Here are other parts of the press release. Source: AGU