This happens again - the number of sunspots is reduced to zero. The day before yesterday, the sunspot group AR2597 disappeared, leaving the surface of the star clear. Yesterday's image of the Sun, taken by NASA's Solar Dynamics Observatory, did not show significant dark cores.
Explanation
Sunspots are areas where strong magnetic fields emerge into the Sun's photosphere. On the solar disk, they are visible as dark spots. Their temperature is about 1500 Kelvin lower than other parts of the photosphere.
On Monday, October 3, a spot AR2598 appeared on the visible disk of the star, which, however, has a simple magnetic field that does not pose the threat of strong flares.
October 3, 2016
SDO / HMI
In total, there were no sunspots for 21 days in 2016. For comparison: in 2015 - 0 days, in 2014 - 1 day.
Promotional video:
What does it mean? Why do spots disappear?
A clear sun marks the approaching minimum of solar activity. The sunspot cycle is like a pendulum swinging back and forth with a period of 11-12 years. The pendulum is close to low sunspot counts right now. Experts expect the cycle to reach a minimum in 2019-2020. From now until that time, we will see the absolutely spotless sun many times. Spotless periods will be measured in days at first, weeks and months later. The current spotless interval is the fourth in 2016.
Explanation
What is an 11-year cycle of activity?
The eleven-year cycle, also called the Schwabe cycle or the Schwabe-Wolf cycle, is a markedly pronounced solar cycle lasting about 11 years. It is characterized by a rather rapid (in about 4 years) increase in the number of sunspots, and then by a slower (about 7 years), its decrease. The length of the cycle is not strictly equal to 11 years: in the 18th – 20th centuries, its length was 7–17 years, and in the 20th century - about 10.5 years.
Explanation
What is the Wolf number?
The Wolf number is a measure of solar activity proposed by the Swiss astronomer Rudolf Wolf. It is not equal to the number of spots currently observed on the Sun, but is calculated by the formula:
f is the number of spots observed;
g is the number of observed groups of spots;
k is a coefficient displayed for each telescope with which observations are made.
Today Wolf's number is 13.
Graph of Wolf number from 2004 (red lines show predictions obtained by different methods). © NOAA
© SILSO
Since the number of solar flares decreases during the solar minimum, many people think that space weather is waning. This is not true. The minimum solar activity brings many interesting changes. For example, when the Sun's extreme ultraviolet production decreases, the Earth's upper atmosphere cools and contracts. This allows space debris to collect around our planet.
The heliosphere is also contracting, bringing interstellar space closer to Earth. Galactic cosmic rays penetrate the inner solar system relatively easily. There is already evidence of an increase in radiation in the Earth's atmosphere, which may be of interest to astronauts and those who fly by plane.
The image below is a graph of the increase in ionizing radiation in the stratosphere, plotted by the organization Earth to Sky Calculus.
Graph plotted using data from a neutron detector at the University of Oulu (Finland):