Background Magic Field

""While typically invisible to the naked eye, high densities of Mist will occasionally manifest in very visible phenomena. The highest concentrations of Mist can even do damage, leading to over-rapid changes in the environment, and violent behavior among animals and those more sensitive to the Mist's effects.""

- Final Fantasy XII

In some settings, magic is more of a raw natural phenomenon than something derived from designated gods or demons, acting as a kind of atmosphere enclosing a planet or permeating a universe. This background magic field, then, is magic in its rawest untapped form as a pervasive energy field that can and does affect the setting in much the same way that radiation or weather might affect our world. In areas where the background magic field is dense, strange things happen, including spontaneous miracles and hideous mutation. However, it can also greatly (if unpredictably) increase the powers of mages who tap into it. Areas of calmer magic are much more predictable, and in areas where the field is weak or absent, magic might not even be possible. If a background magic field exists in a setting, you can be quite sure at least one school of Functional Magic will involve the channelling of background magic into a more focused form. Often it's the sole source of magic in the setting.

Note that while many settings can be assumed to have this, examples should only be included where the Background Magic Field is explicit, apparent, acknowledged, and has tangible impact on the world without anyone even having to do anything to disturb it. This may range from supernaturally altering the geography and climate to enhancing or inhibiting Functional Magic by its very presence.

The Background Magic Field may be composed of, or easily turned into, Mana. Currents in the Background Magic Field may lead to Ley Lines. Said field will usually be detectable, resulting in Supernatural Sensitivity. See also Place of Power, Minovsky Physics.

The trope name is intended to echo real-world background radiation, which captures many of the trope's aspects in a less fantastical way.

If the Background Magic Field has a will of its own, then you are dealing with a Sentient Cosmic Force.

Anime and Manga

 * The magoi in Magi Labyrinth of Magic, which comes from the rukh. Magoi causes all of the world's natural occurrences like storms. Most magic users can only harness the magoi within their bodies, but magi can tap into the magoi and rukh outside of themselves.
 * Reiatsu in Bleach, although this is more of an inner-derived soul force. In Soul Society, where reiatsu is a lot more dense, the characters can use their powers a lot more effectively.
 * Nen in Hunter X Hunter is also a variant of Bleach's reiatsu.
 * Psi in Psyren is yet another... with the  being more conducive to the brain wavelengths.

Fanfic

 * Present in With Strings Attached around the world of C'hou. With proper training, a resident can learn to manipulate the Field to perform magic. Also present but rare are those with a natural genetic gift to manipulate the Field without training.
 * Also present in Sailor Moon Z; largely explored in the episode where Hotaru gets a little too close to Saturn for her own liking.

Film

 * The Force from Star Wars. While the Force is probably the Trope Codifier for a pervasive magical field in Western minds, its fundamental influence is more strongly felt in the Expanded Universe.

Literature

 * Larry Niven's The Magic Goes Away was about what happens when a world of magic users hit Post Peak Oil by using up their Background Magic Field. One character invents a device that signals the depletion of magic in an area (by EXPLODING!) called the "Warlock's Wheel." (It's a flywheel with two spells on it; "spin faster and faster," and "keep it together." When the area runs out of magic, "keep it together" fails and centrifugal force makes it fly apart.)
 * Magic in the Discworld, including the unstable magic areas where crazy things happen. In fact, the Disc would fall apart without this trope. Bonus points for actually being a very close analogy to radiation in some books.
 * Discworld does a fair amount of this. The Unseen University Library is the greatest collection of magic tomes on the Disc, so efforts must be made to avoid the magic going critical. The Science Of Discworld mentions that the last place to try splitting the thaum (the basic unit of magic) is now a very large crater, and anyone who explores it suffers weird consequences.
 * The Wheel of Time has the True Source, a sort of pervasive invisible magic that is available almost everywhere, except in Ogier steddings and the city of Far Madding, which block access to it through natural phenomena and ancient relics respectively. Magic itself, known as the One Power, is channeled from the True Source.
 * In the Age of Legends, there existed something called the "Standing Flows" which enabled a variety of magic-powered technology, such as telephones and airplanes, to be usable by non-mages.
 * The Heralds of Valdemar series. Not only do ley lines run everywhere, but in places where they get muddled, Bad Things tend to happen. Like people and animals mutating, sometimes even overnight. Naturally, the Kingdom of Valdemar backs onto a whole wood full of muddled ley lines and mutated creatures, one of the remnants of the catastrophic war that occurred over three thousand years ago.
 * Magic in The Acts of Caine series exists as "Flow," which magical adepts can channel for their own purposes.
 * The Recluce saga has the Order/Chaos Balance, which can be manipulated by those sensitive to it, but exists in everything and alters the way that the laws of physics and thermodynamics affect ordinary matter, weather and climate, technology, living organisms, and geography in that universe.
 * In Piers Anthony's Incarnations of Immortality, Magic is the result of a "fifth fundamental force", expressed through particles called "magicons"-- the Magicon field of Earth has an effective radius not much larger than Earth itself, so magic cannot travel between planets.
 * And in his Apprentice Adept series, magic is a natural force generated by Protonite/Phazite, a substance formed in the vicinity of a certain kind of Negative Space Wedgie and found abundantly only on Proton/Phaze; the Functional Magic itself only works on the Phaze side and where the two worlds overlap.
 * Magic energy in The Dresden Files is drawn from a Background Magic Field. Magically-closed circles will "seal" the enclosed area off from the rest of the Field, limiting the magic that is available to magic-users trapped within (and keeping purely magical beings from crossing in or out). Magic energy will instantaneously flow back into the drained area as soon as the circle is broken.
 * In Harry Potter, areas of intense magic such as Hogwarts cause electrical devices to stop working.
 * The Tradition in Tales of the Five Hundred Kingdoms is an ambient magic native to the land and acts like the laws of nature. Specifically, it causes people's lives to follow the fairy tale they most resemble... without regard to how nice, happy, or even feasible those stories are.
 * The different types of fae from the Coldfire Trilogy--earth, solar, tidal, and dark--are this, with an emphasis on volatile.

Tabletop Games

 * In the Greyhawk campaign setting of Dungeons and Dragons, magic is considered to operate this way. While there are gods of magic, they are more defined as deities with supreme knowledge and mastery of magic rather than being the source from which it emanates.
 * Magic in Forgotten Realms is derived from the Weave (or the Shadow Weave), which is mostly all-permeating. In wild magic zones it's "snarled" or mildly damaged, and places where it's severely damaged or suppressed are dead magic areas (both can be repaired, but it's hard). In last tens of centuries it's also rather broken as a whole.
 * During the century-advance for the setting to reach 4e, one of the goddesses of magic died, and vast areas of the Realms lie now under a dense area of wild magic, called the Spellplague, that either makes you very, very sick, before mutating into a terrible monster, turns you into a terrible mutated-monster right away, or gives you cool spellscars that come with interesting powers. In 4e FR, players may optionally be allowed to start with one such scar, and there's a theme (an optional PC element equivalent to a low-level Prestige Class that you get on top of your other class for free, much like Paragon Paths and Epic Destinies in 4e are to Prestige Classes at higher levels) called the Spellscarred Harbinger that grants multiple spellscars over the course of the first ten levels of play.
 * In Eberron, the background magical field is extremely stable, especially as compared to worlds like Krynn and Toril. Magic is so reliably abundant that that it has actually been commercialized. The nation of Cyre was destroyed mysteriously during The Last War, and where it once stood, a terrible mist of wild magic now stands, and no one goes in there now except for clans of Warforged, crazy villains, and crazy PCs. However, this had no effect on the stability of magic beyond Cyre's borders. There are also areas called "manifest zones" where the planet Eberron is magically linked to other planes of existence. In these locales physical laws may become a fusion of both worlds. For example, the city of Sharn stands within a permanent manifest zone that enhances anti-gravity magic and allows for otherwise impossible towers and floating structures.
 * In Ravenloft, the supernatural Mists form the boundaries of the Core, Clusters, and Islands of the world, and likewise manifest within these territories at the whim of the Dark Powers. New lands can congeal from the eerie Mists when a new domain is created, and domains whose darklords have been destroyed may disperse into Mist as if they had never existed.
 * The Dragonlance setting, however, mostly subverts this. The availability of "primal magic" is directly connected to the amount of Chaos active in the world. The more Order the gods imposed on the world the less available this magic became, until Chaos reasserted itself. Otherwise, Rule Magic powered by energies furnished by the Gods of Magic, embodied in the planet Krynn's three moons, is the primary source of arcane power. While one could technically utilize this system of magic without swearing allegiance to one of the gods, if the gods are absent then this kind of magic does not work anymore.
 * Wholly averted in the Dark Sun campaign setting. The world of Athas has no ambient magical energy at all, which means that all magic has to come from a specific source. Divine spellcasters draw upon elemental forces. Wizards draw upon the energy of plants (and sometimes animals and people) in quantities relative to how powerful the spell is. Only psionics really come from an undefined source, although they are usually attributed to being an expression of the user's own personal energies.
 * In Shadowrun, this is the Earth's aura, or rather the combined auras of all living things on the planet. Mages have a hard time in space.
 * Also in Shadowrun, areas of highly aspected magic to a particular (usually toxic) element are rated by their background count. The higher the background count, the tougher it is on the mage. (2 is discomforting, 3 is Nausea, 4 is utter revulsion, and 5 is called a Mana Warp.)
 * GURPS has had local Mana levels since at least 3d edition, there are even advantages/disadvantages allowing a character to alter the mana level in their immediate area (Yes, they're expensive). The Celtic Mythos source book also had rules for wonders generated by wild mana levels. Aspected magic and mana are also covered.
 * Ley Lines and Feng Shui are also mentioned in GURPS source books as being representable by local Mana levels.
 * The 3d edition Religion source book also has comparable variable levels of "sanctity" for a particular religion or specific god.
 * Both Warhammer universes. The results of high background magic can cause anything, from random mutations and sponaenous deposits of Green Rocks, all the way to a downright Negative Space Wedgie.
 * Exalted has Essence, the energy that makes up and gives form to Creation. Every person has a little Essence in them, but the Exalted (and a few Heroic Mortals) are capable of channeling it to full effect.
 * Rifts uses this idea as the foundation of the magic used in the setting, concentrated into "Ley Lines" that criss-cross the landscape like glowey highways. Magic is boosted near these lines and starts getting a bit wonkey where they cross each other.
 * In Mage: The Ascension reality is composed of quintessence, the "thread" of the "fabric" of the tapestry of the world. Everything is suffused with quintessence to varying degrees, and can have different Resonance, or magical impressions based on how the world perceives that object or person. For added fun, mages can make use of the Sphere of Prime in order to manipulate quintessence and restructure the magical nature of people and objects around them, including shutting off the flow of quintessence to such things, effectively erasing them from existence.

Video Games

 * The Mist in Final Fantasy XII provides the page quote. It's an unstable, intangible and (usually) invisible field, which impacts everything from geography to technology.
 * This is even carried over to game mechanics. The rate at which your party's MP regenerates depends strongly on the thickness of the Mist in the area.
 * Mist also carries over to Final Fantasy Tactics a 2, which also takes place in Ivalice. It doesn't play as much of a story or gameplay role, though there are some battlefields where the "weather" is a thick Mist concentration, which lets Geomancers use their Mist Storm spell.
 * To a lesser extent, Final Fantasy IX's Mist, too. It's less dramatic in its wildness, but still responsible for the shape of travel and civilisation on the Mist Continent, as well as its hordes of ravenous beasts.
 * This is actually a battle mechanic in Chrono Cross. The magic field is always of a certain elemental type, and can be influenced through spellcasting. The (elemental) color of the magic field determines how powerful magic of that type is, and in order to use Summon Magic the field needs to be the specific entity's color.
 * Arcane magic in War Craft setting is energy from the Twisting Nether that is channeled to the mortal world through leylines. Mages tap into this energy to cast spells. Warlocks draw their power directly from the Nether (and by making pacts with demons that live there). Shamans, Paladins and Priests rely on being granted power by deities or elemental spirits. Its somewhat unclear how Hunter magic works, and Hunters using mana could simply be a case of Gameplay and Story Segregation. Death Knights use some sort of runic system that channels leyline energy through predesigned patterns to create effects, rather than using the Death Knight's personal will.
 * Hunters using mana is entirely for game design convenience. Back in the alpha they operated off of focus, a resource more similar to energy and it has now been brought back for Cataclysm.
 * Aer in Tales of Vesperia is one of the main elements present in the atmosphere of their world. Anyone with the proper magical bracelet and a basic knowledge of how to do so can cast magic using aer.
 * This is how Mana works in Tales of Symphonia. The source of it is a Mana Tree (or the seed of one), and it's concentrated around the homes of the Summon Spirits, although the cause and effect of this is not entirely clear. Either way, the strength of this Background Magic Field is a central point driving the plot of the game, since everything will die if it fades completely, and the Seed that provides mana to the two worlds at the start of the game only provides enough to supply one of them...
 * Blaz Blue has Seithr, which is made of particles of the Black Beast. It is the power source for the Ars Magus, but causes harmful effects in high concentrations. Unfortunately, it has completely saturated the surface of the planet, meaning humanity is forced to live at high elevation.
 * Magic (which was unconnected to seithr) already existed before the Black Beast, but only a handful of people could use it. Ars Magus was invented by a magic-user as an alternative system so that more people could fight the beast.
 * Dragon Age combines this, Spirit World and Dream Land in the Fade.
 * In The Elder Scrolls, Magicka is energy that is bleeding off into the material plane from the plane of Aetherius where the gods reside, through the plane of Oblivion. The sun and stars are holes in Oblivion, leading to Aetherius.
 * Occasionally mentioned in Kingdom of Loathing.

Webcomics

 * In Drow Tales, Mana is a Functional Magic - like cloud of energy generated by the Life Force of the Fey races (elves, fairies, drow, etc.) and is necessary to sustain their immortality. It fills the cities where they live, and it can even be seen by drow but is invisible to humans.
 * In Tales of the Questor (and to a lesser extent, Quentyn Quinn, Space Ranger), Lux is another natural field, explicitly compared to heat, light, electricity, sound waves, atomic forces, and gravity, generated by living things. Most humans can't sense the field or how it is manipulated, but other species like the Rac Cona Daimh can sense and use it, and many fairly simple physical phenomena manipulate the field. A barrel, some salt, and a copper pipe is turned into a Lux bomb that blows anyone nearby off their feet and supercharges (or explodes) nearby magical items, stone 'wells' are used to store the stuff, tapers can suck Lux out of other items, and a properly tuned antenna can use Lux flow to guide a dirigible. Density of the field changes how hard it is to cast spells, how quickly magical items recharge, and can even act as a light. In a reversal of how the trope normally runs, areas rich in Lux are lush and lively, while those places with weaker fields seem blasted and strange to the protagonist.
 * In one filler arc of Elves With Mecha, one character complains that the Steampunk world he all-of-a-suddenly finds himself in has no Magical feild Harmonics.