Our Expert Voices conversation about the 2017 solar eclipse.

The solar corona — the outer atmosphere of the Sun that is revealed so spectacularly during a total solar eclipse — is one of the great mysteries in space science. The biggest puzzle of all is its extreme temperature of several million degrees. Why is the corona so hot when the solar surface below is a comparatively paltry 6,000 degrees?

Most solar physicists believe the primary source of the heat is stressed magnetic fields that permeate the corona and impulsively release pent up energy when the fields reach their breaking point. Occasionally this produces giant explosions called solar flares and coronal mass ejections. But much smaller nanoflares are happening all the time — about a million per second across the Sun — each equivalent to the detonation of a 50 megaton hydrogen bomb.

Why it matters: The corona produces large quantities of X-ray and ultra-violet radiation that are absorbed in Earth's upper atmosphere. Variations in radiation affect the propagation of radio signals, which adversely affects communication, navigation, surveillance, and precision weapons guidance systems. The upcoming solar eclipse affords us a rare view of the Sun's atmosphere and a chance to study how it gives rise to nanoflares so that we can improve space weather forecasts.

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Trump commutes Roger Stone's sentence

Roger Stone arriving at his sentencing hearing on Feb. 20. Photo: Drew Angerer/Getty Images

President Trump on Friday evening commuted the sentence of his longtime associate Roger Stone, according to two senior administration officials. Stone in February was sentenced to 40 months in prison for crimes including obstruction, witness tampering and making false statements to Congress.

Why it matters: The controversial move brings an abrupt end to the possibility of Stone spending time behind bars. He had been scheduled to report to prison on July 14.

Updated 10 hours ago - Health

Which states have set single-day coronavirus records this week

Data: COVID Tracking Project and state health department data compiled by Axios; Map: Danielle Alberti and Naema Ahmed/Axios

13 states this week surpassed records set just last week for their highest number of coronavirus infections in a single day, according to the COVID Tracking Project and state health department data. 16 states in total reported new highs.

The big picture: The United States' alarming rise in coronavirus cases isn't just due to increased testing — particularly where the number of cases has grown fastest over the last month, Axios' Andrew Witherspoon and Caitlin Owens report.

Updated 10 hours ago - Politics & Policy

Coronavirus dashboard

Illustration: Sarah Grillo/Axios

  1. Global: Total confirmed cases as of 6:30 p.m. ET: 12,389,660 — Total deaths: 557,279 — Total recoveries — 6,830,596Map.
  2. U.S.: Total confirmed cases as of 6:30 p.m. ET: 3,169,611 — Total deaths: 133,885 — Total recoveries: 983,185 — Total tested: 38,856,341Map.
  3. Public health: The reality of the coronavirus bites.
  4. Trade: Trump says he's no longer considering phase-two trade deal with China because the pandemic damaged the two countries' relationship.
  5. 🎧 Podcast: Rural America has its own coronavirus problem.