An angry sun at the center of our solar system from time to time blasts the planets with solar flares, proton hailstorms, and magnetic tsunamis. These aspects of space weather are the phenomena that come from the sun. The remainder of the often confusing assortment of space weather terminology describes how the Earth is impacted by the light waves, particles, and magnetized plasma that the sun occasionally launches in our direction, delineating myriad different consequences for our lives and technology.
Of course, we not only want to know what weather is, but also how it may affect our lives. Knowing that rain is made up of water droplets falling from the sky is important, of course, but we really want to know if an unusually potent rainstorm will lead to swollen rivers, broken levies, and flooded homes. Likewise, when a billion of tons of magnetized plasma is hurtling towards us from the sun, a magnetic tsunami, we really want to know if this space weather phenomenon will trigger a major magnetic storm, potentially inducing massive over-voltages in the power grid and thereby causing a widespread blackout. Here we will explain how the primary space weather phenomena—solar flares, proton hailstorms, and magnetic tsunamis—impact the Earth and potentially disrupt our lives.
Fortunately, the Earth’s magnetic field and atmosphere protect humans and other biological life on Earth from being directly impacted by the harmful effects of solar flares, proton hailstorms, and magnetic tsunamis. Nonetheless, these carriers of the sun’s mass and energy to the Earth can adversely affect the space environment around the Earth, potentially playing havoc with the advanced technologies upon which we depend daily, including electrical power, intercontinental air travel, satellite communications, and GPS navigation. On Earth, of course, our terrestrial weather changes from day to day, hour to hour; one day may be pleasant with plenty of sunshine and a light breeze, while the next day is racked by the dark, threatening skies that foretell a coming thunderstorm. Space weather varies from one day to the next as well, so let’s see what makes a day of fine weather in space, and what happens when a stormy sun shatters that calm.
The Warmth of Sunlight upon your Skin
On many days, the surface of the sun is relatively quiet, with no sign of the sporadic bouts of violent activity that can launch extreme space weather towards the Earth. On such a fine day in space, the sun bathes the solar system with the steady, warm glow of its light. Physics dictates that any object emits a spectrum of electromagnetic radiation that is determined by the object’s temperature. Electromagnetic radiation is a general term that includes a wide assortment of different waves, from radio waves to microwaves, from infrared light to visible light to ultraviolet light, and from x-rays to gamma rays (here listed in order of decreasing wavelength, or increasing energy). On a quiet day, the sun’s spectrum of light is not much different than a giant light bulb in space glowing at around 6000 degrees Celsius. Most of the sun’s radiation emerges as visible light, containing every color of the rainbow. A smaller fraction of the sun’s energy emerges as infrared light (like the warmth we feel from a campfire) and ultraviolet light (why we need sunblock on a sunny day). A vanishingly small amount of the sun’s energy arrives in the form of x-rays, which are quite thoroughly absorbed high in the Earth’s atmosphere before reaching the surface.
The spectrum of electromagnetic radiation
The ultraviolet light from the sun is also largely absorbed by ozone and other gases in the atmosphere, often with enough energy to kick out an electron from a neutral gas atom or molecule, leading to significant ionization of the upper atmosphere during the day. All of the ions and free electrons created by this process form the Earth’s ionosphere, which shrinks, but does not entirely disappear, at night time. This layer of ionized gas is essential for long distance high-frequency (HF) radio communication, helping ham radio enthusiasts to beat the curvature of the Earth by bouncing radio waves off of the ionosphere, enabling communication with distant locations beyond the local horizon.
Although the surface of the sun, called the photosphere by scientists, is a toasty 6000 degrees Celsius, further up in the atmosphere of the sun, known as the solar corona, the temperature rises rapidly to reach scorching temperatures of over a million degrees Celsius. How the solar corona is heated to such phenomenally high temperatures remains a mystery to scientists, one of the longest standing questions in our exploration of the sun and the solar system. In fact, the heating of the solar corona is such a fundamental question that NASA has long pursued plans to send a spacecraft mission to the sun to measure it directly. These plans are finally coming to fruition with Solar Probe Plus, a billion-dollar spacecraft mission to sample the outer reaches of the solar corona, due to launch in 2018.
From the Mind of a Scientist Comes the Solar Wind
Although hazardous storms on Earth—thunderstorms, hurricanes, blizzards, and tornados—may seem massive to a family cowering in the relative safety their home’s basement, on the scale of the planet Earth these ferocious events are confined within the very thin layer of the Earth’s atmosphere. Above the center of the Earth sits about 4000 miles of iron, molten lava, and solid rock. Resting upon the Earth’s surface, the atmosphere extends just about another 60 miles further, just 1.5% of the radius of the Earth. From space, the atmosphere looks like little more than a thin shell of blue haze. All of the fury of a hurricane extending over a thousand miles across the ocean is encased within the lowest layer of that 60~km thick shell.
(Left) The thin blue haze of the atmosphere above the Earth’s surface seen from the International Space Station. (Right) The thousand mile span of Hurricane Katrina over the Gulf of Mexico.
A fascinating fact is that the solar corona (the atmosphere of the sun) is different from the atmosphere of the Earth in a fundamental way. The nitrogen, oxygen, and other gases that make up the air in our atmosphere are held in a state of rest by Earth’s gravity. Although winds andstorms may push those gases across the surface, they remain in a relatively stable set of horizontal layers. In 1958, a young physicist at the University of Chicago, Professor Eugene Parker, used mathematical equations to deduce that the ionized gases (or plasmas) that constitute the solar corona cannot be in a similar state of rest. Instead, the million-degree solar corona must be constantly blowing away from the sun, flowing out into space, ultimately reaching supersonic speeds. This theoretical prediction of a “solar wind” was greeted by the world’s foremost astrophysicists with derision, but the Soviet Luna 1 spacecraft, launched one year to the day after Professor Parker first submitted his findings to the Astrophysical Journal, confirmed the existence of this supersonic flow of plasma emanating from the sun.
Indeed, the solar wind is now known to blow away from the sun in all directions, varying in speed from around 600,000 to 1,800,000 miles per hour. The solar wind is a hot and very tenuous plasma, an ionized gas made of mostly hydrogen and helium, basically a collection of protons, electrons, and alpha particles (alpha particles are nuclei of helium atoms stripped of their electrons, each containing two protons and two neutrons). The temperature of the solar wind plasma is about 100,000 degrees Celsius, but its density is extremely low, with only about 10 protons in one cubic centimeter, about the volume of one of a typical pair of playing dice (not the big fuzzy kind found on car rear view mirrors in the 1970s, but the kind used to play a game of Monopoly). Compare this to the density of the air we breath, which has about 10 billion billion molecules of air in that same size playing die. This solar wind blows steadily through the solar system on a calm day in space, reaching out beyond the planets to the outer edge of what is known as the heliosphere, the realm of influence of our sun in the Milky Way Galaxy.
Illustration of the solar wind blowing from the sun, impacting the protective magnetic field of the Earth.
Magnetic Fields and the Aurora
But the protons, electrons, and alpha particles that make up the solar wind do not flow alone. The solar wind plasma drags along with it the magnetic fields that emerge from the sun. This magnetic field does not always just come along quietly for a ride with the solar wind, but rather it can stir up a bit of space weather action even under calm conditions.
As it turns out, a magnetic field cannot typically generate a plasma, and likewise a plasma cannot pass through a magnetic field. This peculiar fact is why the solar wind must drag the embedded magnetic field along with it, and also why the Earth’s magnetic field can serve as a protective shield against the million-mile-per-hour gales of the solar wind. But when one plasma with its embedded magnetic field impacts another plasma and its magnetic field, the relative direction of the two magnetic fields influences the outcome. The magnetic field of the Earth is oriented so that the magnetic field above the surface points toward the north pole, providing the basis for centuries of navigation by humankind. If the remnant of the sun’s magnetic field embedded in the solar wind is also pointed toward the north, then the solar wind plasma will simply spread out and pass around the Earth’s protective magnetic field. But if the magnetic field embedded in the solar wind happens to the pointed instead toward the south, those two separate magnetic fields can merge together in a process called magnetic reconnection, enabling the plasma from the solar wind to penetrate into the Earth’s protective magnetosphere.
This process of magnetic reconnection sweeps magnetic field lines from the side of the Earth facing the sun (where it is day time) all the way to the other side of the Earth away from the sun (where it is night time), piling up the magnetic field on the night side. Eventually, enough of the magnetic field can pile up on the night side of the Earth to cause the magnetic fields there to undergo further sporadic bouts of reconnection more than 40,000 miles above the Earth’s surface. This subsequent reconnection of the magnetic fields releases the built-up field at the night side back into the flowing solar wind. When this occurs, the magnetic field lines connected to the Earth undergo an abrupt shift. This shift travels along along the Earth’s magnetic field down towards the surface as a particular kind of magnetic wave, called an Alfven wave, just like the bump that travels along a garden hose if you quickly yank it up and down. When these Alfven waves reach an altitude of about 6,000 miles above the surface, they can accelerate a small number of electrons to incredible speeds of nearly 40 million miles per hour. These electrons speed down the magnetic field lines towards the Earth, and when they reach about 100 miles in altitude, they begin to collide with the molecules in the much denser atmosphere below. The collisions excite the air molecules, causing them to radiate away the energy of the collisions as the dancing lights of the aurora.
Illustration of magnetic reconnection occurring on the night side of the Earth, triggering the glowing of the aurora.
If magnetic field in the solar wind remains pointing southward, this process of sweeping the magnetic field from the day side to the night side of the Earth will repeat itself every three hours or so, with repeated occurrences of the aurora dancing in the skies of the polar night. Scientists call this process a substorm. It is somewhat analogous to a period of scattered thunderstorms on a summer afternoon in the American Midwest. On a sunny afternoon, the wind picks up as the skies to the west darken. Before long, the winds howl, heavy rain pelts the ground, lightning flashes, and thunder rolls across the plains. Not long afterwards, the storm eases to a cold light rain, the thunder and lightning fade into the distance, and the clouds eventually cede to the emergence of the hot summer sun, evaporating the recent rain, returning to a hot, and often sticky, summer afternoon. And, if another scattered storm is headed your way, the process will repeat itself. Substorms are the space weather equivalent, breaking up a fine, calm day in space with a predictably choreographed magnetic dance far above our atmosphere, a fraction of its energy channeled into a dazzling auroral display.
The Van Allen Radiation Belts
A final important characteristic of near-Earth space that persists, even on quiet space weather days, is the system of Van Allen Radiation Belts. The Earth’s radiation belts were first discovered in 1958 by Professor James Van Allen of the University of Iowa with the Explorer 1 spacecraft, the first United States mission to orbit the Earth (the third man-made object to orbit the Earth, after the launches of Sputnik 1 and 2 by the Soviet Union, which sparked the Space Race during the Cold War). Not significantly different from the magnetic field due to a bar magnet (which you may have seen with the trick of using iron filings to reveal the magnetic field lines), the Earth’s magnetic field emerges from Earth’s surface near the south pole, directed northward along longitudinal lines (like the thick black lines on basketball, or the boundaries between the slices of an orange), spreading out over the space surrounding the Earth, only to converge again as it passes back down through the surface near the north pole. Scientists call this basic shape of the Earth’s magnetic field lines a dipole geometry, somewhat similar to the shape of a cored apple.
(Left) The magnetic field of a bar magnet revealed by iron filings. (Right) The dipole magnetic field of the Earth.
As it turns out, high-energy protons and electrons can be trapped in this dipole-shaped magnetic field, collecting largely in a ring around the Earth’s equator, at an altitude of ten to twenty thousand miles above the surface, forming the Earth’s Van Allen Radiation Belts. In addition, these trapped protons (and other positively charged ions, like oxygen) drift in a westward direction around the Earth at a speed of about 25,000 miles per hour, encircling the Earth in roughly an hour; the negatively charged electrons drift to the east, at about 10% of the ions’ speed, encircling the Earth about twice a day. Together the drift of these ions and electrons forms the ring current that encircles the Earth. Because Maxwell’s equations of electromagnetism tell us that a flowing current will generate a magnetic field, this ring current modifies the Earth’s magnetic field. On quiet days, this ring current is relatively steady, not leading to any significant changes of the Earth’s magnetic field. But, as we will see, during periods of stormy space weather, abrupt changes in the ring current can disturb the Earth’s magnetic field, leading to significant changes of the magnetic field, even down at the Earth’s surface, potentially disrupting the electrical power grid and causing widespread blackouts.
Schematic of the Earth’s Van Allen Radiation Belts.
And Then the Weather Turns . . .
Now that we know what constitutes a pleasant day of weather in space, join us next time as we learn how the Earth is affected when space weather turns ugly.